Honestly, if someone is using the x-card (sufficiently explained as a way of avoiding content that someone is not comfortable having at the tabletop) as a shut-up card, you've got a pretty clear sign that's someone you shouldn't invite back to your table, they've obviously going to be overbearing and hard to work with.
Suddenly it's a way to vet new players on top of the existing aim of filtering out uncomfortable content. So many features!

