I saw that there is an instrument known as the 'Superbone'. With the slide of a trombone and additional valves like a trumpet, you'd think you would have the best of both worlds. So why is this instrument not used very much?
Let me offer some alternate perspective besides the tuning issue referred to in the other answer.
- They're awkward to hold. The situation when operating the valves with your left hand is worse than a bass trombone: there's about the same (or more) mass in the valve section, and there are fewer points of contact for you to support the instrument.
- They're expensive and uncommon, which makes it difficult for folks to start playing them. Only a couple serious instrument companies (Holton and Conn) made them, neither in quantity, and no such companies produce them today as far as I'm aware. Used models from Holton and Conn still sell for fairly high prices today - $3,600 for this example or $4,000 for this one, for instance. Even copies by less-trusted manufacturers are already somewhat expensive: $1000 for this example, which I wouldn't recommend buying unless you have a thousand dollars to throw away.
Most trombonists would have started on a slide trombone. With which, like the violin family and voices, can play in any temperament, not only 12tet. This won't happen with a valve trombone, using the valves.
I've played with several valve trombonists, and the main reason they used them was physical problems - bad shoulders, elbows, wrists - probably from playing slide trombones for years. Decent quality valve trombones don't come cheap.