Well, it depends on how common you make them. In the setting - at least the version I "grew up on" - there are indeed some people who, by virtue of their innocence/indoctrination, have a certain resistance against Corruption. That the system makes little difference between them and some Hive ganger who grew up eating half-cooked rats and killing kids for their lunch just doesn't seem very fitting to me. In short, I don't believe everyone should start on the same level (0) on the Corruption scale, and for some few types of characters it may indeed seem awfully unsuitable to become tainted at all.
I agree that this is very much a matter of interpretation, though!