I don’t have a problem with her interpretation of “defy”, I just don’t agree that she actually established in the text of “Philosopher’s Stone” that they had turned him down. I’m perfectly happy to accept that they did so on the basis of her word–it’s her world, after all.
On that basis, the OP’s question is valid. My answer is what I alluded to in my previous post: she would have been useful. A big part of Voldy’s schtick was manipulation; he said himself that he’d always been able to charm those he needed, and he wasn’t just talking about spells. While he clearly had a particular hatred for muggles, I think he played up the “pureblood” angle as one way to control his followers, giving them a target. After all, he wasn’t pureblooded himself, and neither was Snape. His personal feelings, I think, were subsumed by his obsessions with immortality and power; if a muggle who had never even heard of magic could be useful to him in those pursuits, he would use them, then discard them–just as he would use and discard a pureblooded follower.