I find amusing that people are so woefully uneducated about biology that they think that EVERY biological mechanism has some sort of evolutionary advantage. Just because an organism has a fit into an environmental niche, doesn't mean that it does so to maximum efficiency. It fits, it thrives, and it can do all that even with allergies and hemophilia and color blindness.
People who insist on evolutionary reasons for every biological mechanism are just betraying their deep-seated need for a Designer. In other words, they're closet religifags. Their WANT for a Designer is so strong that they refuse to comprehend the randomness and lack of purpose built into biological systems.