E.g. in the extreme case, with three candidates, suppose everyone hated the other two options so much that they refused to even write in a #2 choice.
Of course, this seems like a completely silly argument, because it's much more likely to generate majorities than the single-choice system.
Personally, I don't mind if someone wins with 43% of the popular vote. I don't even mind FPTP much, and yes, I mostly vote third party. But from other people in the state, that 50% number was a concern, and I don't think this addressed that adequately.