This is a very good question. My X will be the first car I've ever owned that didn't have a spare. I've been thinking that I would get the Tesla tire repair kit with the sealer and compressor for a temporary fix, but I don't like the idea of having someone unfamiliar with Tesla repairing the tire and dealing with the sensor and the sound deadening in the tire. A spare would be better, even if just to carry during a long trip.
But there are other issues to consider. You'd also need a jack and tool (can't remember the name) to remove the lug nuts, and can you even jack up an X? And if so where? I just looked in the manual and it mentions nothing about how to replace a tire yourself, jack points, etc. The manual just mentions the repair kit and calling for roadside assistance (in the unlikely event that you happen to be within 50 miles of a service center).