Gazoinks: I do this in pretty much every strategy game, as I like to play by making a bunch of allies and then sitting around making cash. "What's that, buddy? The British are invading? I can't hear you over all my money!"
AFnord: It does come back and bite you from time to time though. In a recent game of Victoria 2, I ended up promising to support my ally in a war against a roughly equal powered foe, but decided to not to do anything. Well, my ally lost a big battle, and was soon crushed. My newfound enemy then decided to move against me. It was not pretty :( Also, it would seem like not actually fighting the enemy results in you getting a lower relationship with your allies in CK2.
Still, it is going pretty well for me, Ireland is united & England burns, everything is going just as planed.
Yeah, it definitely depends on the game. In the Total War games I like to do a variant of this by fighting my neighbors first and either weakening or making them vassals. Doing the turtle strategy in Alpha Centauri got me crushed though, because I didn't expand enough.