If you have to ask, then you probably don't need it. This is true of bodies, lenses or anything else. If you're having a problem with the equipment you've got and there's something out there that will help fix it, then go for it. Being a Nikon guy I can't comment on Canon specifics. If you were shooting sports professionally and kept missing your shot because of slow AF, then maybe it's good to look at a different body. If you shoot often hand held in low light, then maybe an IS lens will help - you could shoot at f2.8 with non IS or maybe 5.6 (or higher) with IS for a little more DOF.

It's a question that only you can answer.