Bard's are essential in W7 as far as I'm concerned. The thing with putting an enemy to sleep is that you basically have a 100% hit chance against them, their defense is reduced WHILE they're asleep, and they're not going to be attacking you in the mean time. Yes, there's magic that sleeps enemies too, but early on it's a lot less accurate and more expensive.
I don't think I used a bard in W6 so maybe less necessary there, but still I can imagine them being super useful. You DO tend to have a higher base hit rate in W6, though, which helps.
Also I THINK Ninjas and Bards have some thief skills? You can eventually get the bard's thief skills higher, anyway.