Originally Posted by rowuk

The answers are quite simple
1) a long slide doesn't leak air or water easily My Prana trumpets have a short slide
2) the long slide design is old and allows in many cases, tuning your trumpet to concert A instead of Bb I can't imagine Mr Monette wanting me to tune my B flat trumpet to A
3) there are a lot of players with extreme tension issues and bad breath support OK but what about your last line
4) a long tuning slide is cheaper to build mechanically stable and reliable than a short one What about the prana short slides and My Monette wasn't cheap.
5) There are a variety of standard tuning frequencies from A=435Hz to A= 446 Hz throughout the world. Don't they buils the trumpet to what you need?
6) intonation changes with air pressure and altitude.ok

To get back to the original post: generally, when you start getting lessons and your breath support starts developing, you pull the slide OUT.Why? Once your breathing is at an advanced state and you start addressing body issues, you can sometimes push the slide IN. The position of your slide is not a measure of playing expertise! What about #3 above