Question said:Most PCs are medium sized. Using a large great sword jumps the damage from 2d6 to 2d8 or 2-12 to 2-16 damage, on average a 2 point damage increase.
With monkey grip that is a -2 penalty to attacks. With power attack you could subtract 2 points from your BAB and gain +4 damage, on average twice as good as simply using monkey grip.
There's no reason you can't do both. But I agree it's a slightly silly feat. It would work better combined with some additional means to remove the -2 penalty for an oversized weapon.