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There is plenty of main tuning slide on the 2XJ tubas and they tend to play pretty well in-tune with or without the bit.

That leads me to believe that using a bit coincides with my personal experience that it was furnished for ergonomic reasons.

I have a friend who plays a Conn 12J that will not be in tune unless he uses a tuning bit. If he doesn't use a bit the tuning slide is not long enough to get down to 440.

Shave and a haircut...

Sorry, that's two bits.

I think the tuba police are going to come after you! But I think they are for both ergonomic and tuning. From my understanding, Conn's had short leadpipes specifically for you to put the bit in and bring it to pitch while being able to adjust it to the most comfortable playing position for you. My Conn plays in tune without it though so I don't use one usually. The main reason is because my front action valves are a stretch to reach with the extra length from the bit. It's uncomfortable. I would use one all day long on a top action 2XJ though!

I play my Conn 25J fairly regularly. I experience the normal intonation tendencies either with or without the bit. From my experience, after consistently practicing the Conn 2XJ tubas the past few years, the "flat" F isn't really an issue. Also, the Eb, E, and F in the staff can be high, so I use 1&3, 2&3, and 1&2. The Conn 2XJ tubas definitely take some work to play, however they are definitely worth the effort if you're are looking for "that" type of sound.