Most people I know don't get their blood type checked at birth.
Anyway, probably the reason the O people are getting hounded by the Red Cross is because O blood means you are a universal donor - any blood type can take your blood, but Os can only take blood from other Os. It's because the A, B and AB blood types indicate the presence of A, B, or both A and B antigens, while O means you don't have any of those antigens. And you can't take blood from someone who has any antigens your blood doesn't have.
So for an AB like me, it's the reverse - I'm a universal receiver. Since my blood contains both possible antigens, I can take blood from someone of any Rh-positive blood type, but only the 3.4% of the U.S. population who also is AB+ can use my blood. And since they can also use O+, A+ or B+ blood, my blood isn't particularly valuable. (But that doesn't mean that ABs shouldn't participate in blood drives! I would if I wasn't so squeamish about needles.)
(I've also heard the universal donor/receiver thing is the reverse with plasma donations - can anyone else clarify this?)
Is your point that AB are a useful type to donate? Or not really?