The Dark Lord was famous for not distinguishing between pureblood and halfblood houses.  All are equal at my feet, he would croon, but Severus knew as well as anyone that this was due to strategy, not some kind of belief in equality. If only purebloods were able to follow the Dark Lord, his army would contain less than a hundred soldiers. Pureblood lines were thinning with each generation, each progeny sicklier and weaker than the last. 