[TPIN] Transposing "Carmen"

[Achias1 at mchsi.com]

We're well along on rehearsals for "Carmen," and they are going well, although I do sweat a bit on the A 
transpositions.

Since the opera is written somewhat equally for B flat and A horns, I'm using my B flat for everything 
even though it is an orchestral setting.

My question: Why in the world are there parts for B flat and A anyway? In analyzing the part, I can find 
no reason why the part switches from B flat to A throughout the opera. In at least one place, the part is 
in A for a bit, then switches to B flat using the same (transposed) pitches. Why?

The orchestral part says "Trumpet" at the top and contains B flat and A parts.

The two movements for stage trumpets (one in B flat and one in A) are written for trumpet or cornet, 
but since they are bullfight fanfares, it seems that they should be played on trumpet.

Where is the sense in this?

Dave