> That's bogus

To expand a bit, it's a perfectly valid use, but most people will forgo having an extra word that has no role in the phrase.

"Il est médecin" and "Il est un médecin" are perfectly equivalent so people will choose the shorter.

The "un" comes back when using the noun as an adjective isn't possible anymore, e.g. by adding an actual adjective "Il est un bon médecin"