Finding out your parents were BSing you about Santa can be very traumatic to kids. Little kids tend to consider their parents to be nearly god-like in stature and power, finding out they were lying about something is a pretty heavy hit on the core concepts of reality for a kid like that.
I don't ever remember seriously believing in Santa, something about not having a chimney and having multiple relatives that worked in department stores and thus seeing multiple department store Santa on the same day caused my BS detector to tingle for what was probably the first time in my life. And my mom was never much of one to try to snow the kid under, she figured life was complicated enough without making her kid suspicious, so when I asked her about it she spieled on the spirit of giving and stuff like that, basically saying that no there was no Santa the person but Santa the concept was still good and important only with words a 4 year old could grasp.
If I had kids I'd be very vague on the Santa front, there are too many possible really bad repercussions to lying to kids about something when you know they'll see through it eventually.
It just really doesn't have to be that complicated. When questioned, I was told if I wanted presents I should believe - end of conversation. The trouble is all the overwrought explanations.