Below Average--despite appearances and the initial "kewl d00d; I can wield 2 1eet mercurial greatswords at once d00d" reaction, this feat is not advantageous for pure damage dealing capacity.
Wielding a bastard sword and shield and then power attacking for 2 is both more flexible (because you don't have to power attack if you don't want to) and yeilds slightly more damage than wielding a greatsword and shield. Wielding a longsword and power attacking for 2 gives you only half a point less and costs one feat fewer (assuming that most fighters take power attack anyway).
Dual wielding a pair of shortswords and power attacking for four will yeild more average damage than dual wielding greatswords at the same attack value. And the character is more flexible, doesn't look like a munchkin, and doesn't look stupid. Plus nobody asks him how he manages to carry two shortswords, a dagger, a mace, and a composite longbow. Some DMs might just ask that of the greatsword dual wielder.
The area that monkey grip can be useful is for reach weapons and other weapons that have advantages other than pure damage. A fighter built around disarming might find it advantageous to monkey grip a heavy flail. (As he'll come out two points ahead in a disarming contest even after figuring the attack penalty in). Similarly, a character could monkey grip a glaive, longspear, or spiked chain and use a shield at the same time. All of these characters would actually be giving up average damage in order to gain a tactical advantage.
However, I think there are other feats which are more efficient at giving tactical advantages like that and don't share monkey grip's drawbacks.
