DE15 and DA15 both exist and are pretty commonly used, I'm not aware of any other conflicts in normal versions.

However, you can get weird Dsub connectors with things like COAX in there, so having the shell sizes have names can be useful.

Coax and high-current/voltage pins are not that weird. You can also get truly weird semi-proprietary pins like fiber optics or even pneumatics/fluidics.