I almost always leave the previous version installed. Sometimes it's handy to be able to produce .veg files that other users of the previous version can still open. Sometimes it's handy to try out something in the older version to answer question in here. Sometimes i just want to see how the old version worked differently than the new version to convince myself i hadn't quite gone completely crazy yet.

It's particularly useful with Sound Forge since you can't have multiple sessions open simultaneously. If i'm in a live situation and need to do some hack and slash editing to produce a CD as quickly as possible, i can record each song in Sound Forge 7, then edit that song in 8 while 7 is recording the next song. I suppose i could also record in Vegas and edit in 'Forge, or the other way 'round, but i like using 'Forge for both recording and editing. It is a pity that SONY doesn't allow multiple instances, but having two different versions installed gets around this limitation.