The point is that you may not need as much protein as you think to build and preserve lbm. Another point is that protein overload can cause health problems as gjohnson pointed out. Your body can only use so much protein, and if you're not getting the other macronutrients necessary for energy/protein synthesis, your body uses the protein as energy and gets used to destroying the protein instead of utilizing it properly. Basically, your body can only use so much protein properly to build muscle anyway, so why overload it if all it's going to do is cause health problems?