Grade inflation. Blame No Child Left Behind.
Strange, I don’t recall making any such post.
As an O-neg, I used to get called all the time by three blood donation agencies. (My 8-week donation exclusion from one agency wasn’t known to the others.) So, if you consider getting unsolicited calls a disadvantage, then that’s one for having O-neg. 
On top of being O-neg, I also haven’t been exposed to some virus that most people have been. (It’s innocuous to adults and older children, but dangerous for newborns.) So my blood is especially useful in NICUs.
Hey man, zombies need blood too. I think.
that’s cytomegalovirus. i’m free of it too, as well as a B negative donor. it’s a wonder i have any blood left. when i turned 40 i got a tattoo on my ankle for my birthday. in indiana, and i assume all other states as well, there’s a standing year-long moratorium against donating when you get tatted.
to the day the moratorium expired, the bloodsuckers called me to come in. 
here’s the dope straight from the horse’s mouth (American Red Cross).
Blood Types and the Population:
Caucasians African American Hispanic Asian
O + 37% 47% 53% 39%
O - 8% 4% 4% 1%
A + 33% 24% 29% 27%
A - 7% 2% 2% 0.5%
B + 9% 18% 9% 25%
B - 2% 1% 1% 0.4%
AB + 3% 4% 2% 7%
AB - 1% 0.3% 0.2% 0.1%