I find that Bmin is far, far more useful than Bmaj - all my D/Gs have Bmin. It's brilliant for a passing chord in both G and D and, as such, is used far more than a Bmaj is needed. As Greg says (but the other way around) if you need Bmaj, just play the B bass note.
Also useful for a few tunes in B minor - much easier to play on a D/G than B major.
I'm more interested in making the home keys of the melodeon more interesting than attempting to play in keys which lose the inherent logic of the fingering - not saying it can't or shouldn't be done, just that I don't really do it.
Use that Bmin to colour your tunes!
Martin