No disadvantage at all in having multiple versions installed. There is no 'interference'. If you have completed v8 projects you should retain v8 - because you may find problems later if you need to make some changes. The general principle is 'edit with the version that created it'.
The only circumstances I'd suggest removing it is if you are chronically short of disk space.
Insanity is hereditary, you get it from your children
As Neale states, having several versions is usually no problem at all. One user, ATR has 7, 8 and 9, IIRC.
The only thing that might be of a concern would be Organizer, and its catalogs. I assume that each version contains its own version of Organizer, so there should be no conflict there - but that is just my assumption.
I cannot recall anyone with issues, when they have more than one version coexisting on the same machine.
I also agree with the advice to finish Projects in the version started. While there is usually good forward compatibility, say PrE 8 Project to PrE 9, there can be glitches too. It is just best to finish in the version, where the Project was started - that eliminates ALL possible glitches.