There's a problem with that, long-term, in that the previous versions will one day (one day soon? Lion?) be incompatible with the OS. I have this situation with 60-odd projects written in VisualStudio6, and another 20-odd in VC8.
VC8 is kind enough to perfectly convert VC6 projects if you want to move up to the newer IDE & compiler (well worth it BTW) but it's way more convenient to fire up VC6 for a quick fix on an old project.
All of which was fine until I had to get Win7 and guess what - VC6 just won't run on Win7, and several upgrades are now required to each of these legacy projects.
So now I either *have* to convert them all or run a virtual machine with WinXP so I can use VC6... it's a hassle any way round. I wish now I'd kept my old WinXP box - but then I don't get the choice about that kind of thing :(