Based on the pathogen hypothesis at the end of the article, it seems to me that diversity in blood type itself could be the key benefit to a species here.
Some scientific data re. various cancers and their correlation with ABO groups:<p><a href="http://ispub.com/IJPA/13/1/5982" rel="nofollow">http://ispub.com/IJPA/13/1/5982</a>