The original idea of Tuba Christmas (and keep in mind it's called "TUBA Christmas" for a reason) was to provide for an ensemble of conical bore low brass instruments. The invitation on the web site says "Tuba and euphonium players are again invited to participate in one or more of the TUBACHRISTMAS concerts/events presented throughout the world." It doesn't say "low brass players" or "brass players". In fact, for what it's worth, the Wikipedia page on Tuba Christmas lists "conical bore" as one of the requirements. And this requirement is repeated on a number of other sites. I don't know whether these are all copying from the Wikipedia page or the Wikipedia page is copying from somewhere else, but it's safe to say that there appears to be a near universal view that Tuba Christmas is for conical bore instruments in the "tuba family". Alto horns definitely count. I"m not sure about flugelhorns (though, morphologically, I would count them).
Trumpets, cornets, French horns, trombones? No. Serpents? Tough question. I think they should be grandfathered in -- and have played in Tuba Christmas events where a Serpent has appeared. Ophicleide? I'd say no, but I can see the controversy.
In full disclosure, I have in recent years organized small ensemble performances playing out of the Tuba Christmas book. Some of these have been euphonium/tuba groups and others have included trombones. But these aren't official Tuba Christmas events. It works with the trombones. It's okay. But it doesn't sound as good -- not to me anyway. And there's just nothing wrong with restricting these official Tuba Christmas events to conical bore instruments. It's the sound that Harvey Phillips was interested in, and it's something special. If you need to use a trombone to play that music, by all means do so. But maybe use a bit of common sense and consideration, and don't show up at a Tuba Christmas expecting to be welcomed into it.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)