Why not place 2-channel speakers BEHIND listener?

I have a home office in an attic where I'd like to set up a 2-channel speaker system. My desk is against a wall that slopes in (so that I can't place speakers in front of me against the wall). I have some nice bookshelf speakers, but they are too big to place on the the front wall. I was thinking of placing them behind me, about 5 feet apart and facing toward my listening position. Or I could just buy very small computer speakers to sit on my desk. Any advice?