The idea is that which person/party is going to vote for would matter in a general election because if you’re a say a tricky fraudulent Dem/Hillary supporter, you don’t want to bother committing fraud by voting as someone who would have voted for Dems/Hillary anyway.

The primary fraud idea is somewhat silly - if you end up stuck with a ballot on the other side, what’s the point really? You don’t have a horse in that race. In either case, knowing the affiliation of the fraudee is important for the potential frauder.