I believe they're useful, and I don't want to hurt anyone's feelings nor discouraged sales.
The only thing about adding something like this on to an instrument is that sometimes (whether or not deservedly so) some things can function as red flags - another such thing being a main tuning slide trigger.
In my experience (and for my personal tastes) the mouthpiece shouldn't sit too far back from the choke point, and not very much of the reverse taper receiver should be exposed. Most tuba mouthpieces feature a thickness of metal on the back end (mostly for practical purposes - to define them as more dent resistant back there), whereas - if shaved down (in the style of Schilke) or even bored out to a sharp point - when they are bored out to a pretty sharp point and the exit bore of a mouthpiece is about .525", it seems like it works out pretty well for a mouthpiece to sit back a really short distance and then encounter a choke point (origin of the mouth pipe tube) that's roughly that same inner diameter (and maybe even regardless of the size and length of the tuba (though I seem to prefer F instruments' mouth pipe tubes originating at closer to only around .500").
terrible run-on sentence
(but I'm not going to fix it)
Finally, (here goes. LOL) being that I don't see much purpose for tuba mouth pipe tubes to start much larger than .525" - and I also don't see much need for a space between the end of the mouthpiece and the choke point being much more than 1/8 inch to 5/16 of an inch (and though I offer euro shank mouthpieces for sale) I don't see much use for euro shank tuba mouthpieces.
It takes some craftsmanship (not all that much), but (unless they are rotten) European rotary tuba mouthpiece receivers which are formed from the mouth pipe tube themselves can be re-tapered (once the cylindrical overpart is removed). For other instruments, regular receivers that accomplish what I described above are what I prefer. When I mouthpipe tube's small end is larger than what suits my tastes (measurements listed above), I can build some more taper and a smaller choke point into a (non-adjustable) receiver appliance itself.
This whole post response is a disaster - as far as wording is concerned, but - if I don't go fix some horns - Mrs bloke is going to beat me.
