Old thread I know but interesting.
I think I can bring something to this discussion.
Firstly no trumpet really needs the adjustable third slide, all musicians have the ability to lip notes into tune.
The cornets and trumpets from 1850 through to 1900 and beyond had no adjustable third slide and players simply learned to lip the notes that were out of tune.
Secondly the mouthpiece horn combination affects how lippable the notes are, I slipped a jazz mouthpiece into an instrument that centred notes well with other mouthpieces and suddenly the notes were sliding all over the place, playing a good pitch became difficult but with effort was achieve-able.
Thirdly the Martin Committee has a reputation for being easy to lip and the notes dont centre on this instrument as strongly as on other instruments.
Moreover an orchestral player does not want an instrument that is hard to centre the notes, his or her task is difficult enough without having to expend extra effort just to play in tune.
I dont think you will find many Martin Committees in orchestras for this reason.
The committee however is perfect for jazz players with its easily bent and lipped notes but it takes a strong and talented player to master the Committee, It does not surprise me in the least that Kehaulani has mastered the Committee.
To answer the question then, trumpets are already made so that the adjustable third slide is unnecessary, if you can handle them in that manner, but beginners, players with weak embouchure, players who play legit, orchestral players, all these players need easily and strongly slotting instruments that have an adjustable third slide in order to play well and to meet their playing needs.
Fully compensating instruments have been made in the past of course as already mentioned, that slot well and dont need an adjustable third slide to do it, but we jazz blues players need the ability to slide the note roll off it fall off it and gliss so we need the committee and instruments like it that make those embellishments easier.
I would suggest that if a player has an aspiration to play jazz then the ability to lip notes and dispense with the adjustable third slide is a necessary skill to learn until it becomes second nature to play on pitch without adjusting the slide and then they can choose when to use it like kehaulani does.
It is about becoming a well rounded more complete player.