Maybe dredge wasn't yet what dredge would be; I can buy that.  Dread Return didn't exist yet, so non-dredge mages had a little more time to fight back before being put on their backs.  There was, at the very least, a window to castMorningtide. 

So in the innocent ages, Morningtide was a serviceable sorcery.  The problem, of course, is that being a sorcery, you needed the opponent to give you an opening in order to use it for effective defense.  So if the other player were to flip his or her deck into three Narcomoebas and a Dread Return all in one turn?  No window for Morningtide.  Even if it was just a big dredge setting up the "fair" reanimation of a big Golgari Grave-Troll?  Morningtide might be too slow. 

In the middle of the last time we had a Ravnica block to deal with, graveyard players had a different defensive tool to deal with: Leyline of the Void.  Leyline of the Void had quite a number of things going for it, and you don't have to look very hard to see it played even today in big formats like Legacy as a sideboard card; truth be told, the first article I can remember reading by Luis Scott-Vargas showed the great champion playing a stack of Leylines main deck! 

Like Morningtide, Leyline of the Void was an imperfect solution to a growing problem.  It was great if it was in your opening hand—heck, even non-black players could start a game with Leyline of the Void in play—but in most other games you had Leyline of the Void in your deck, you would not have the four turns necessary to get it onto the battlefield. 

And for that matter? 

If you actually cast the enchantment? 

Leyline of the Void would put a knife into the kidney of a dredge player who hadn't killed you yet... but would do nothing about any cards the opponent had put into the graveyard before that point.  If the opponent merely needed another turn to get you (and had already spent the third turn getting into position to kill you the next turn), you were probably out of luck, even if you had the hoser in your hand and the lands to deploy it. 

I, for one, would have considered playing Rest in Peace for just the Leyline of the Void effect... but luckily we don't have that option!  If you are playing against a pure graveyard opponent—say a graveyard combo deck à la Dredge, Cephalid Breakfast, or Reanimator in Legacy—it is more potent and more persistent than a Tormod's Crypt as a "permanent" source of defense.  On the play, hitting this on turn two should be fast enough, and on the draw, it will still be fast enough the vast majority of the time, especially if you are backing it up with some other form of disruption. 

What might not be immediately obvious is that this card completely turns off the popular creature Tarmogoyf, and permanently, unless the opponent directly answers it.  Is it ludicrous to play an enchantment that turns off all the opponent's Tarmogoyfs, even if it doesn't "kill" any?  I think it could be reasonable to turn such a storied creature into basically a terrible chump-blocker.  Maybe your deck is particularly vulnerable to Tarmogoyf; you might consider paying to actually remove a single Tarmogoyf... why not to disable every Tarmogoyf the opponent draws?  That is, when Rest in Peace hits the battlefield, its "Morningtide" ability empties both graveyards (reducing any existing Tarmogoyfs to 0/1)... and due to its "Leyline of the Void" half, they just aren't going to get any bigger. 

Nimble Mongoose?  Werebear?  Mystic Enforcer?  One of those creatures is a bit more popular than the others at this point, but considering how tough a 3/3 Nimble Mongoose is for some decks to deal with (tough to fight, tougher to target), setting all the geese, all game, to 1/1 should be able to give you a huge advantage... or at least take away the opponent's existing advantage. 

And flashback?  All off, essentially all game (at least until Rest in Peace is itself removed).  You can't necessarily "turn off" the front side of flashback cards (that is, opponents can still get rid of your Rest in Peace with a Ray of Revelation from the hand), but no straight-to-graveyard shenanigans are going to get you, and at the very least you have transformed any and all Snapcaster Mages to flashy Coral Merfolk. 

Rest in Peace's combination of emptying the graveyard first and then remaining vigilant against future graveyard-filling makes it almost uniquely capable of fighting against "fair" graveyard strategies.  You probably wouldn't (and shouldn't!)  even consider using Tormod's Crypt as a strategic weapon against Tarmogoyf and Nimble Mongoose.  You don't want to commit a card for not-a-card, especially when the opponent can just refill.  Similarly, the past year gave us any number of showdowns between Nihil Spellbomb and Snapcaster Mage, but in that case, Nihil Spellbomb generally replaced itself by drawing another card, while Snapcaster Mage—body remaining or no—was essentially a one-shot in terms of graveyard-based advantage.  Rather than such a one-to-one relationship between threat and answer, Rest in Peace is a single point of defense that doesn't actually kill anybody (okay, maybe Mortivore), but can reduce many supposed super-threats to a level of bungling mediocrity. 

Now, on the subject of (relatively) fair fights, consider our hero against what looks to be the Level Zero of the impending Return to Ravnica-imbued Standard format: Zombies.  Rest in Peace might not seem like the be-all and end-all for the forces of the undead, but it is certainly worth a moment of consideration (remember, just a few weeks ago, the Magic Internet was ablaze with differing opinions of whether a different non-removal enchantment might be appropriate in combating the little Æther Vial); I have always considered Geralf's Messenger the particular troublemaker in terms of Zombies threats, although Gravecrawler (especially upcoming, and alongside the unusually durable Lotleth Troll) is what gives Zombies much of its staying power.  At the very least, even at the expense of a card, Rest in Peace should make it possible to play—and win!—an attrition war against Zombies in Standard.  Add up all the half-cards Zombies doesn't get via Gravecrawler re-buys and undying bonuses and you have a pretty palatable return on investment. 

Some things to keep in mind when deciding whether or not to run Rest in Peace relative to other options:

That said, Rest in Peace can still be bounced; for example, dredge decks have used Chain of Vapor in the past as an anti-Leyline card. 

Although it should go without saying that if your opponent Chain of Vapors your Rest in Peace but then fails to kill you, Rest in Peace is not only more economical to re-play, but hits Morningtide-style on the way down (again).  But yeah, your opponent's goal is going to be to kill you. 

On the subject of being a permanent—and not just any permanent, but a white enchantment—Rest in Peace makes a perfect buddy buddy with existing tool Enlightened Tutor (Enlightened Tutor is already played in WU or GW decks, for instance as a Sylvan Library-finder). 

For all its virtues—and clearly there are many reasons you might want to play this card—Rest in Peace has one substantial drawback: It's symmetrical.  Leyline of the Void was a card that graveyard-based decks could themselves use against other graveyard-based decks.  Rest in Peace, on the other hand, will turn off your Tarmogoyfs, take the edge off of your Snapcaster Mages, and remove the ongoing incentives currently attached to your flashback spells... which really merely says that decks that play these kinds of cards might want to choose a different answer but those that don't have quite the new tool. 

Subtly, in Standard, returning dual lands like Temple Garden and Hallowed Fountain mean that light splashes for white can give blue or green decks access to this exciting tool.  So if you were banking on some kind of Unburial Rites/Frites as an early option for the upcoming Standard?  Or you think Burning Vengeance might just get the day in the sun it never did last year?  Perhaps planning on accumulating a bunch of +1/+1 counters "for free" via the Golgari scavenge mechanic in or away from the existing Zombies shell?