Given that the mail clients that I've used tend to fill out To and Cc
nearly at random, I think there's already so much noise involved that
trying to extract any information from whether someone is on the To or
Cc lines is a lost cause *for replies*. I agree with you much more
for original messages, where the sender has addressed the message by
hand.
I generally find that to be the case also - To and CC are far more
meaningful on original messages than on replies.
But in the vein of trying to make email a more effective communications
medium, I'm hoping we can encourage MUAs to improve in this regard.
(so much communication is now being done by email that it's kind of
important that it work well)