I think party systems should actually be standard nowadays :P
But, yeah, party systems make playing online a much better experience. You cooperate well with friends making the game more fun, and you don't need to worry about getting into a game with frustrating teammates that you can't work well with (or betray you. Or just suck in general :P).
Currently playing: Modern Warfare 2 (360), Borderlands, Metroid Prime Trilogy, Guild Wars