The bass tuba is typically seen (here, in the US) as a solo instrument. It's never recommended to be played elsewhere other than maybe quintet.
However, I think that the bass tuba is the instrument that for me (and a lot of others) makes the most sense to own. Here's why.
I'm a teacher with no intention of auditioning for a major symphony or band. I DO play in a local "semi-professional" group that's basically equivalent to a higher collegiate ensemble. That group has 7 tuba players including me. For fun, here's what we play: B&S Pt-6, Pt-6P, MRP, Rudy 5/4 C, MW Thor, M&M Hirsbrunner BBb clone, Yammie 641. A lot of firepower there. This is the only thing I do that I "need" the MRP.
So, for what I do, I probably don't need such an expensive instrument in the MRP. I don't even really need TWO tubas. But I own them and have no intention of getting rid of them

Here's the thing: For what we actually do in music school, I think the bass tuba is the better choice- particularly one with a decent lower register. Think compensating Eb, 5v Eb, piston F type horns. Those bass tubas that are really quite versatile.
What do you actually do in music school? Or, rather, what are you required to do?- Most programs tell you that you need ensemble credit- one large ensemble per semester. Then, you go to lessons and play etudes and solos and might participate in a brass quintet or other chamber ensembles. For me, I would've been able to do all of that on my big Willson F I use now.
So often there are other players with the big horns anyway, and unless you really want to continue and audition, you can play most everything on a large bass tuba and probably spend less money in the long run. Nobody likes college debt- why not put that money that you saved from buying the 6/4 you don't need into your tuition fund?
Yes, there is certainly merit to getting really good at the big horn, I totally get that. But, how often do you see people buy this York copy that is much too big for 99% of the stuff that they do, only to get the hand in band and sound particularly bad on most standard solo repertoire that just sits better on F and Eb?
Once you leave school, you'll likely find a community group to play in and might do church music/quintet work. All of which would be a great fit for the F/Eb tuba.
I think this should be the FIRST tuba one buys, rather than a secondary, more akin to what they do across the pond where comp. Eb is the main horn for most things. I just think it works better for most things we do. So much of your music education on your primary instrument is dedicated to solo playing where, if you have access to an F or Eb, opens up a huge library of stuff that's just not easily playable on the big tuba.
I hope my point comes across a little bit through my rambling- what do you think?