I find the X-Card most useful at conventions. When I sit down at a table I have no idea where strangers' lines are. It provides a more frictionless way to let people tell you how to be courteous, without knowing them very well.
I think guidelines and an equivalent of a trigger warning is a better solution. It's really hard to modify a campaign on the fly if for example someone is uncomfortable with cults but that's the primary driver of the storyline.
