The only reason en passant exists is to stop pawns from advancing 2 squares - beyond an opponent's advanced pawn - thus shoring up the lines. Pieces move differently than pawns, so it's not really an issue with them; the main idea is so that the captured pawn can't "escape". I imagine that without en passant, chess games would be a lot duller and feature a lot more closed positions.
It's an interesting idea you have, but I don't think there's any need for pieces capturing en passant. Advancing a pawn beyond a piece is not a permanent case of "passant" as doing so with another pawn. Once the pawn advances beyond the reach of an opponent pawn, it's done so permanently; a piece can always move back where it came from.
I hope that makes sense :P
So I had an interesting position in a recent game and it brought up a question that I have never thought about before. Why is en passant only allowed to be between pawns?
Basically I had a bishop on b5, he played a5, and I wasnt sure why I could not play Bxa6, as anywhere else on the board, the pawn could only move 1 square- which is why the move exists in the 1st place. But why is it only for a pawn to take en passant? Shouldnt it be allowed by any piece that the pawn tries to escape from by making the double move?
This may sound like a silly question, but think about it, why shouldn't it be possible? Im a pretty decent player, but this is the 1st time the issue has crossed my mind, so I was hoping someone had a clue.