Beastmaster as a Subjob

(this guide is a work in progress.)
What are the advantages to using BST as a subjob?
The main draw here is that your Charm success rate is determined by your BST's main job level, not its current subjob level. So if you have BST leveled to 75 and try to Charm something as WHM20/BST10, you have the same chance to land the charm as a Lv20 BST. (This doesn't include differences in CHR.)
Fighting monsters with a similarly-leveled pet by your side is a huge advantage at lower levels, effectively increasing your kill speed by 50-100% depending on how strong your pet is.
Your pet will draw hate and take a few blows every so often, further reducing your downtime. This again depends on the relative strength of your pet.
You can send a pet into a swarm of linking mobs to break up a spawn. You can just let it die and then charm a new pet. Only the mob that you had your pet attack will come after you.
What are the disadvantages?
First of all, BST is an advanced job, meaning you need to first get another job to Lv30 and then do a short quest to unlock it. The quest itself takes place in Jeuno and Qufim and doesn't require any fighting (aside from getting someone to kill the Kraken to be on the safe side).
In order for your Charm to be effective, you must level BST as high as you need it to go. 75 will obviously be the best but not everyone wants to go that high. This qill requite time in itself, but if you like to level jobs then your investment will pay for itself in the long run.
The only useful pet command you will have available at low levels is Fight; you won't even be able to Heel until main level 20. You also cannot use Call Beast to summon your own pets when BST is subbed. Finally, you won't be able to dump pets with Leave until 70, so you'll need to deal with pets uncharming.
Because your CHR will be lower than an equivalent BST main for most jobs, you will need to deal with more frequent mischarms.
That being said, the strengths of having a second "party member" that doesn't hog any EXP usually outweighs the downsides.
When is subbing BST a bad idea?
BST sub works best with jobs that already have a form of healing available, such as WHM, BLU, RDM, or even BLM and DRK with their Drain spells.
If your main job has no healing available to it, BST can still work fine if played properly, but you need to watch your HP. Don't forget to get buffs from a Field Manual, and open every Treasure Casket you find.
BST sub isn't as effective when your main job already has a pet available, since you can't have more than one pet out at a time.
Why BST? Can't I just sub another pet job?
When it comes to pets and subjobs, for the most part, BST easily wins out. Compare it to the other pet jobs when subbed:
/PUP - Lets you summon an automaton at half your main job level. You can still use the basic Lv1 Animator and Maneuvers, though.
/SMN - Only allows you to summon Carbuncle with halved Summoning skill.
/DRG - Call Wyvern is not available when used as a subjob.
At VERY low levels (1-12, basically the 3 nations' newbie areas), I'd say /PUP is actually more viable than /BST due to the decent strength of the automaton vs. your enemies, and powerful abilities such as a 100 HP Stoneskin and a beastly Regen rate. However, it will quickly be outpaced by /BST as your level increases and you move to new areas.
This is great and all, but why should I even bother soloing?
Square-Enix made so many changes to the game since its release to make solo play far more worthwhile than it had ever been.
- Rings which grant EXP bonuses (including the fantastic new Allied Ring for players Lv30 and above).
- Increased experience points from killing EP/DC enemies.
- Fields of Valor, providing hourly EXP/Gil rewards for killing various enemies, as well as Regen/Refresh/Protect/Shell/Reraise and various food buffs.
- Treasure Caskets, which spawn randomly upon defeating enemies and produce temporary items that restore your HP/MP/TP and remove status problems.
- Random weekly bonuses called "Super Kupowers" which grant certain special effects under Signet, a few of which really help when soloing.
Add everything together, along with the damage output of /BST, and you can easily see 5-6k/hour at lower levels. That's not bad!
Soloing will never come close to the EXP/hour of a great party, but there will be times where you can't be bothered with the hassle and setup of forming and gathering an EXP party, or you only want to log on for an hour, or you want to work on another job while farming, or you're sick of terrible Dunes parties. In any of these cases, you could certainly do far worse than subbing BST and laying waste to the local fauna.