It's a philosophy, not a requirement. Mint thinks you should upgrade Ubuntu that way as well.

I just keep separate partitions and do a full install on the ones I need. No sweat.

On the other hand, I did a "ubuntu" style upgrade to 10.04 LTS, and that worked fine, too (I run that on a netbook for their netbook interface). It took longer than a full install on Mint (perhaps being a slower machine had something to do with it), but it worked out ok.

Ok, but if they eventually strike out 'on their own', it's not something they're going to test, potentially leaving you in the lurch.

I imagine we'll at least be warned.