I've always carried a spare, and don't remember ever needing to use it. OK, once, back in my hot rodding days. Did a really smokey burn out to impress the ladies and ended up having to throw a spare on after melting the tread off a tire. That wouldn't count, would it? Other than that, never used one. I do know that I take two spares for the 'Bourbon when we head over to the basalt rock areas in Eastern Oregon, just in case. I run Toyo MT's on it and they tend to cut pretty easily with a heavy rig loaded down. Haven't needed to use them yet, but they are there just in case.
Sure as shootin', as soon as you don't have a spare you'll need it in the worst possible way in the worst possible place. That's coming from a belt and suspenders kind of guy. I've carried crap in my rigs for years that I've never used. With the way most people wheel, and where they wheel, new tire technology and everything else taken into consideration, a spare tire is a nice to have and not required for most of us.
On the rational side, no spare removes a lot of weight from the back of the Jeep. Probably over a 100 pounds by the time you factor in the wheel, tire, and carrier. That is not a trivial amount on a 3,500# rig.