So what if one of those groups happens to be full of vicious mercenaries? They'll help him learn how to fight! And sure, another group might be made up of "courtesans" (see: ladies of the night), but they'll teach him all about sneaking! And another group is full of ... well ... you get the gist, right? The game's handful of factions will assist Ezio as he murders his way through Renaissance Europe.
One final thing: even if you don't care about each group, make sure you stick around for the surprise treat nestled after the end splash screen. You'll thank us.