(Also, how compelled might a very particular player be to use that slide to tune any note, such as open pitches which might suffer, has so many latest/greatest instruments have been found to be really challenging to play in tune)?
There are two tubas whereby I do not pull slides:I'm not a slide puller
One of them is my F tuba. It plays remarkably well in tune, features six valves, and the so-called slots are barn door wide so that any minor lipping has no audible effect.
The other one is my 3 + 1 compensating E flat tuba from 1958 with the detachable recording bell. It is also very flexible and slippery, and there are no slides that I can easily reach, and I'm never playing it louder than to supply a bass line or a tuba solo in a small jazz combo, so anything from very soft up to regular forte is pretty easy to move, as far as pitch is concerned.
Not sure what percentage of people understand what the person quoted just above meant, when he said this.I pull slides for tone, and not for tuning.
What he was trying to explain is that he can probably lip most any pitch in tune, but it's so much nicer to play pitches on an air column that is just the right length for that pitch, because so much more resonance occurs...
...and I'm going to stick my neck out and say that whether that particular person realizes it or not (and surely they do) it's much more likely that they're going to play a pitch in tune when the tubing is the correct length. I believe we all TEND to become accustomed to out of tune pitches on our instruments and tend to accept them and eventually they sound perfectly fine to us, and this would probably include fifth partial pitches and third partial pitches which may be flat, as well as a fourth partial 2-3 pitch which is so often flat, second partial open pitches which are often sharp and 6th partial pitches are often sharp with 8th partial pitches often being flat. I believe we can even become accustomed to the sound of our out-of-tune pitches grinding against and an in-tune bass trombone and/or an entire ensemble. Since I don't believe that I can hear the grass grow, my number one emphasis in practice time is to play in tune, and to know exactly where slides optimally belong for every pitch on a given instrument. With my F and E-flat tubas - whereby I do not pull slides, I know just how flat or sharp various pitches lie on those instruments without correction. Finally, I tend to default tune to just under optimal tuning, because it's so much easier to push pitches up than to relax them down, particularly when playing at volume level extremes at either end of the loudness spectrum.
Good tone with good pitch is wonderful, but good tone with bad pitch is just bad.