I don't know that USM is really necessary but it is nice. I only own one lens that has it (I'm a Nikon guy so different terminology) and actually it's the lens that probably needs it the least (Sigma 10-20). It's probably more beneficial with longer focal lengths because the focusing ring has to turn more when focusing. That Sigma lens probably travels about 90 degrees or less from MFD (minimum focusing distance) to infinity and a 300mm lens might need a turn and a half (or 540 degrees).

There may be other differences between a USM and non-USM lens but that difference alone isn't the end of the world IMO. It will cost more - this means that the focusing motor is built into the lens instead of using the one in the camera body. Again I'm a Nikon guy so correct me if I'm wrong here but I think it's the same as their AF-S lenses. Generally they'll focus quicker, quieter and can be manually focused by just turning the focusing ring rather than switching the camera body to manual focus mode first.

I compared a Nikon AF-S and non-AF-S lens, and these were really the only differences between the two. It was the 80-200 f2.8 when the AF-S was pretty new, and I didn't see a night & day difference in focusing speed. If I was making a living shooting sports I'd have probably bought but I'm not so I didn't. For the record, it was with a pretty fast camera (F100). Since you're using the camera's focusing motor for comparison, the camera focusing speed now can become an issue. Pro bodies will be faster, no doubt about it - but I turned this post into way more of a book than I needed to, and it's not really worth worrying about. It's one of those things like I said before, if the limiting factor of getting your shot depends on focus speed then it's a good time to upgrade to something faster. I'll bet in the real world that it won't be that big of a deal...