The codenames for every major Debian release are named after characters from Pixar's Toy Story franchise. Debian's unstable release is fittingly named after Sid, an unstable character from the Toy Story movies.
I actually like Mint's naming scheme, of alphabetical women's names that end in an a sound. Only one problem: They decided to go with the minor upgrade cycle during Mint 17. The 17th letter is Q. I'm frankly surprised they were even able to think of "Quiana." That's why the rest of the 17s were R names, Rafaela, Rebecca etc. so now they're off by one.
Numbers give the wrong impression that one version follows another. Debian release channels exit alongside each other individually. Giving the release channels names helps to make that distinction. It also makes for an easy layout of packages in APT repositories.
Sid is and always has been Sid. If you were to assign numbers, what number should replace that name? There are perfectly working labels for release channels and there is no reasonable replacement.