Now that I have been playing Principal Trombone for the last 3 years, I found that the Alto is a necessity for all trombonists.  Not only does is make life easy on such pieces as the Creation (which I have performed on both A & T), Beethoven 5, Schumann 3, and especially Brahms 1.  I used to get by doing all of these on the tenor, but I feel I can crush these pieces with the alto with no problem.  Coach Brown is correct that you need to be able to play all the alto pieces on your tenor (in my New York Philharmonic Audition it was required to play Schumann 3 on the tenor), but you need to be able to play the alto equally as well in tune and with the great sound you use on your tenor.  I use the Sterling Silver Glassl and have been very happy with the intonation, ease of play in all ranges, and I love the sound the instrument gets.  My colleagues in the Utah Symphony have told me that the sound I get on the Alto is what they like the best.