If we wanted to measure how fast you knock guys around the base paths, we could use OBP with runners on base. But this is flawed, too. Good hitters hit well when men are on base. They also hit well when men aren't on base. Remember, every out is precious, not just ones with men on base.
Some players get walks and singles, and some hit doubles and homers.
SLG is total bases divided by at-bats. This stat nicely complements OBP, and in fact many people use the two in tandem. It has even begun to take hold among traditional sportswriters. By breaking OPS into its halves, we've got the two most important hitting stats represented in the Triple Crown. In some ways, this stat is as flawed as RBI or BA, but it's also the most recognized part of the game.