yeah, pretty annoying to have the C: Drive cluttered with all those files even if you have your library somewhere else.
I don't even understand why they aren't deleted after install - I never had to repair my library ever.
My guess is that it must be related to handling of support questions, some of which can presumably be more easily handled by Ableton if they can tell users how to reinstall stuff. (I also routinely clean up this redundant stuff from my drive - but at least the installers don't place Windows-only binaries on my OS X install anymore, and vice versa, as they used to do. Go figure...)
More generally, imho this also explains the lack of options / preferences (or even their removal, e.g. the plug-in buffer size setting is now gone in 9.x). Live seems to be targeted at noobs rather than power users, and I'd bet there's much more money to be made that way.