The unique names really help with googleability. 'debian 7' will return more chaff than 'debian wheezy'. "Where's the 7 on this site? Oh, it's the forum page number" and so forth. Similarly I can ask a dev in a mixed shop "is wheezy or precise on this" and it's easier than saying "is debian 7 or ubuntu 12.04 on this". Ubuntu numbers are unusually regular and unique, which helps their googleability, but other versioning numbers aren't.