Chuck(G), AND JUST ABOUT EVERYONE ELSE HERE APPARENTLY, with all due respect, I think you are limiting your conception of the tuba because of your preconceived notions about the instrument.
People doing this, players and composers alike, is the reason the tuba has such a small amount of literature with much of it being hokey and meant simply for humorous effect.
Cases for the tuba as a solo instrument:
It fills a niche that many instruments do not. No other solo instrument can go as low as the tuba. The only reason people dont hear low solo instruments is because composers traditionally have not composed for them.
The tuba has a range that is pretty much unmatched by any other wind instrument. 5+ octaves. Very impressive and a lot of potential there. I think that there is even more potential for range in the tuba, as I continue to surprise myself with what I can squeak out on my CC.
The tuba, contrary to popular belief, can both fill the role of a light instrument and a bombastic and intimidating force. Not to mention everything in between. It may take a little more finesse to play in some styles, but it is definately doable. Look at the french horn. Now that is a REALLY tough instrument. Mechanically that thing is just..... AHHHH. Small pipes with a length comparable to a tuba. Insane. But I digress.
Most if not all humans are unable to double tounge on woodwind instruments. This limits them. The tuba is not limited in this respect. point: Tuba literature is not on pace with other less technically capable instruments. See range point as well. Woodwinds have limited range. They (in my opinion) are often not as expressive as a brass instrument or even *gasp* a tuba. However, they have much more literature. This does not make much sense.
And honestly, who cares if Beethoven didnt write anything for Tuba. I sure dont, because I can STEAL what he wrote for other instruments, and make it sound better than when they play it.
I see the issue as such: Tubas are forced to play a supporting role all the time because thats the way it has always been. Lets have a piece where there is four part writing in the tubas and one boring trumpet part that is divided in octaves. THAT would be refreshing and different, not like this Eric Whitaker October nonsense (dont get me wrong, its a good piece, just not revolutionary) And the way too much watered down john williams crap we see from composers like Jim Curnow.
Okay, this may ruffle a few feathers here, but as a tuba player is the reason you don't care about solos is because you're LAZY and never want to practice your band music? Do you want to be able to play 90% of the pieces that come across your stand on the second or third time you go through them? That means you never have to seriously buckle down and practice. The less solo stuff out there for the tuba the better? Just so we can continue to be lazy and not practice to sound as good as other instruments on our parts?
Tubas don't get solos because tuba players are lazy beer drinkers. Myself included.