Sure, but when you're on the plane looking for your seat and arguing with the passenger who appears to be in the same seat as you, it helps if you don't have multiple boarding passes even if only one of them is really valid.

I can see good reasons why the airline might want to make it slightly tricky (although as you point out, not impossible) for a passenger to be holding multiple boarding passes with different seat numbers. It's liable to lead to passengers getting in a muddle and at some point pulling out the wrong one.