Transposing is an important skill for all instruments. It's true that trombonists get some cases for free if they learn the tenor and alto clefs, but it's possible to learn it without knowing the C clefs.

And even trombonists have to learn how to handle sharps and flats in chromatic passages, so there's a lot more to transposing than using a different clef.

> Tenor clef shows up in modern musicals regularly.

This I didn't know, thanks!