http://hints.macworld.com/comment.php?mode=view&cid=124313

You can still leave the source in iTunes set to the Store,
but avoiding those pages with scrolling images.

This general sort of thing can also be a security/privacy thing. I have had it in mind for a while to see about how to make a super secure configuration of OS X. Such a system would have everything removed that you don't actually use, especially background processes. For example, you may notice that there is always a process running which is called iTunesHelper even if you don't use iTunes at all.

In terms of performance, I suspect that the CPU time is less of an issue than the RAM taken by all these background processes. Of course, it is probably more true that RAM is the deciding factor on a multi-core system than a one or two core machine, which is where you would really care about optimizing performance.