What's so special about Power Attack?

[Dandonk]

PA is very useful for THF, since +10 damage is almost always better than +5 attack.

Also keep in mind which classes have good Base Attack Bonuses (comparing at lvl 20):

Artificer +15
Barbarian +20
Bard +15
Cleric +15
Favored Soul +15
Fighter +20
Monk +15
Paladin +20
Ranger +20
Rogue +15
Sorcerer +10
Wizard +10

As you can see, PA is not that useful for your FvS and Monk as for other melee classes (Barbarian, Fighter, Paladin, Ranger).
But when you have many attack bonuses (some good gear, various buffs, etc.) then PA is still very useful.
As others mentioned, it also depends on which mobs you are fighting.

Ummm....

Fvs get Divine Power, a buff that gives them the full +20 BAB. Monks using monk weapons/unarmed get full BAB also.

Now, these two classes may have to hit issues due to not having as many to hit bonuses from class features (weapon focus feats, FE bonuses, PrE bonuses, whatever), but it has nothing to do with their BAB in itself.