In my most recent game, the French sent a huge army into my (German) territory, with clear intentions to attack me. So, I quicly MPPed with England and Rome, since I didn't want to go down so easily. France attacked anyway. The French army, despite its great size, was slaughtered on my doorstep (not unlike WWI trench warfare - same tech level even), and the alliance turned the tables quickly.
The MPP, though useful at first, became a liability when England started a war with someone else, drawing my technologically backward, economically and politically unstable nation into another nasty dust-up.
I eventually weasled my way out of that war and went on to victory, avoiding any more MPPs. Isolationism seemed a better course, though I did end up smacking several invader civs off MY (Mine, All Mine!!

) continent.
So, MPPs have their place, but there can be a serious downside.
-Dearnen