This comes up a lot in the U2U forums with every new release, and has done so as far back as I can remember.
When an old version is uninstalled after the new version is installed, file associations break and cannot be restored. It seems the old version takes file associations with it on the way out, leaving them orphaned. What makes this an urgent problem is that the CC app does exactly this when "remove old versions" is checked.
When I want to remove old versions I always do that prior to installing the new, and it seems to me the CC app could do that too.
It seems there are several different registry edits floating around, and users recommend one or the other in the forums depending. Which is the correct one? What do I tell people on the forums?
Two recent threads:
https://forums.adobe.com/thread/2415195
https://forums.adobe.com/thread/2547864