I cut the sidewall of a tubeless tire a few weeks ago on a broken beer bottle that I didn't notice hiding in the grass. I was able to boot the 2" long gash with a dollar bill, insert my spare tube, air it up and ride it home.
That's the first time in 2 years I've needed to use a tube with tubeless tires, but I was really glad I had it.
I don't really see any downside to carrying a spare tube.