Clotting is very common. I am not a doctor or a nurse, but I seem to recall reading somewhere that clotting is indicative of high estrogen levels during the formation of the endometrium. Maybe someone with more knowledge on this subject will give you a real answer to your question.
I have never had a period with out clotting. I have asked both gynocologists I have had and they both told me it was normal.
I know of only a few that do not have clots. Not that it is a big topic of conversation, but the subject does come up. It seems like the majority of the women I know do experience them.
MickeyVicki's comment does make sense. I agree with her too, I hope someone can give a real answer or "medically correct answer".
Karin