It's unlikely that there are many exo planets with a moon as big as the Moon. Much more likely to have small ones like Mars. Mercury, Venus and Mars are all great examples of bad shit that can happen to planets: Tidally locked + too close to the sun, rolled over on the side, and finally Mars too small to maintain a molten core and magnetic field. The Moon helps avoid the Venus situation and maybe the Mars problem.

Do you think "big enough moon" is going to be similarly rare to "liquid water"? We're getting better and better at finding planets. Not sure how we'd find their moons though.