Classical players work hard to make their playing sound perfect. Perfection in tone, intonation, dynamics, articulations, releases, posture, breathing, etc. Every note they play has to be perfectly clean and played with conscious intent. They study arcane minutiae from hundreds of years ago in a quest to remain loyal to what they believe is long dead composers' original intent. They look down their noses at jazz players for their sloppy playing and poor tone (but to be fair, legit players look down their noses at everyone

Jazz players care little for technical perfection, and prize creativity, emotion, passion, and musical ideas and development. Their playing is immediate, improvisational, and impassioned, sacrificing technical perfection for instantaneous musical adventures. New ideas, uniqueness, and moving the music forward prevails over the notion of faithfully reproducing earlier works. Indeed, playing something the way it was played 50-100 years ago is to be avoided. Been there, done that. Newness, progress, and individualism are prized. They think classical players sound wooden, sterile, staid, stuck up, and lack facility.
For some reason, trombone culture generally dictates that we land in one of those two camps. This is an argument for blending the classical ethic into the jazz/pop/rock/commercial realm.
I think the acceptance of sloppy playing in jazz tromboning is nothing more than an abject surrender to the difficulty of the instrument. It's "too hard" to play intricate, technical, and improvised passages cleanly, so trombonists have simply given up and claimed their lack of ability to play cleanly as unimportant or, even worse, detrimental to their musical expression. Sloppiness is claimed as a feature, not a bug.
I maintain that 99% of those who excuse sloppiness in jazz/commercial playing would play cleanly if they could, but they can't. So in the same way KFC markets their greasy-ass chicken as "finger-licking good!" to turn a detriment into a perceived advantage, jazz trombonists market their sloppy-ass jazz playing as a feature of the idiom, not a lack in their ability.
Much of what makes trombone jazz essentially unlistenable to both other musicians and the general public, alike -- and the reason why you don't have trombones fronting jazz groups (though trombonists seem to think this is a mystery) -- is the rampant sloppiness. To me, it doesn't matter how awesome your ideas are if you can't play them well. Stumbling through them does not improve them, and listeners hear the whole, not just the part you want them to, and your lack of proficiency on your instrument destroys the listenability for them.
Let me dismiss the inevitable excuses: Yes, intentional and mindful imperfections can enhance jazz expression if used sparingly and for a discrete purpose or effect, but 99.9999% of the time it's neither intentional nor mindful, it's just sloppy. Yes, occasionally a mistake actually makes a solo better, but that is never predictable and it's like catching lightning in a bottle it's so rare. Yes, perfection can be boring and sterile, but only when it is mere technique, alone, without musicality, which again is sacrificing one aspect for another when both are needed.
If I could wave a magic wand, I'd send all jazz trombonists into the practice room and lock the door for five years, and make them learn to play like classical trombonists. But jazz players need to go further. They need to develop their technique and facility to the point where they can play all those great improvisational licks cleanly and musically, and not let slop and garbage detract from what would otherwise be great playing. In essence, jazz players should develop the classical ethic and skills required for good, clean playing of the instrument, and then push that forward into the kind of technique and facility required to play jazz ideas that are routinely played with great clarity by sax players, pianists, trumpeters, guitarists, and the rest of the jazz world.
Keep going with acceptance of all the slop and garbage in jazz tromboning and you'll render the instrument extinct in jazz. It's almost there, already. No one wants to listen to the crap in your sound, they want to hear good, clean music.
If you don't have great facility, simplify your jazz ideas to not exceed your technical ability. Develop your technique to the point where you can play intricate and lightning fast licks cleanly and with the precision of a valved or keyed instrument. Make the overall sound of what you're playing the top priority, and refuse to sacrifice either cleanliness or passion. Make glisses, slop, clipped notes and clams the rare and intentional exception to a vast ocean of clean playing.
And quit trying to convince people that your sloppy-ass technique and crap sound are a feature. You're only fooling yourselves.
Okay, that's my opinion. You may now commence firing.
