Uhm, no. Bb instruments playing in Bb are playing in concert Ab. Since the OP posted this in the Eb Alto forum I assume he is an Alto player, in which case he would be playing in concert Db. Still, I would agree that all scales are important.
Uhm, yes. What I was saying, naturally, is that when the conductor calls out a Bb scale, a Bb instrument is playing in C. Right? Sorry you misunderstood me.
Also, I, like you, assume the OP is an alto player, an Eb instrument. However, I didn't understand him to be asking why the Bb scale was so important to him, personally, or to Eb instruments, or why he spends so much time playing in the relative key of Bb (which I recall seeing rather rarely as an alto player). I understood him to be asking why his director calls out concert Bb so much, why they tune to Bb, why so many pieces are written in concert Bb. The answer to that question, naturally, will not be related to what the Eb instruments think; I'm sure we all know that in terms of the make-up of a school band, the Bb instruments are far more prevalent than the Eb instruments. The answer to his question, then, may be that anything scored in concert Bb will be played in the key of C (relative) by Bb instruments. This is especially true for the Bb soprano clarinet, which not only is the most prevalent band instrument in most bands, and is not only generally regarded as the anchor of band music, but has different fingerings for the two octaves. As a result, any key signature choice for musical students, such as middle and high school bands, would probably be strongly weighted to making the key signature easy for clarinets to play.
I mentioned keyboard instruments because, if this assumption for students is correct, it's actually a mistaken carryover from keyboard instruments. For woodwinds, the relative key of D is easier to play than the relative key of C, so concert C would be preferrable (at least to me).