Its to do with the amino acid spectrum, to cover the bases eat two sources of protein in one sitting ie baked beans and wholewheat toast or add amino acid supps to a single source.
Eggs are pretty much perfect on their own as well as whey.
I beleive that the human body uses aminos for what it needs, when it needs to build hair, skin, bone etc... So does this mean that a complete protein will be used for muscle growth only? No!
Does this blow a hole in the complete protein thing? Not really, its still important to get the aminos as near as poss but dont worry about it being perfect.
This of course goes for the better types of protein ie fish, steak, chicken etc...I eat these on their own but soy, nuts etc I would not have without some other type of protein present