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.
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.