Regressions are definitely one of the biggest risks when merging. Your customers will not appreciate that.
Personally I don’t see what’s wrong with tagging releases in SVN. I understand that it’s a bit like using a sledgehammer to pound a thumbtack into your cork board, but it does the job just fine. And who knows, you might actually want to merge it someday if you discover some crippling bug that affects the 12 most recent versions and a bunch of users are still using the old one. If the tag is technically a branch, then it will save you a lot of last-minute scrambling.
Maybe some SCMs think of tagging as just slapping a label on a particular item in the history. That’s fine too, but to me it’s only a superficial difference. I think it’s an OCD-like impulse, getting upset that there’s some awful dangling branch that will probably never be merged. Who cares - I know what it’s for, that’s why it’s in a “tags” directory.