I carried a spare after I shed a few teeth off the original one on my 2013. Two more seasons went by and I do not bother carrying one at all. Unless you ride a turbo or are super abusive on a sled I think it is a waste to carry a spare.
When mine went it lost 4 teeth, you would not have known if you didn't actually look at it. I rode it not even knowing. The other close call I witnessed was my buddys jackshaft bearing failing leaving a ton of slack in the QD belt. It was ratcheting like crazy, rode it out almost 20 miles.
My experience is that belts after the original 2013 batch are pretty bomb proof. I think I did 30-50 miles each to break on the last two. No problems and I am a pretty good sized guy who rides aggressively.
I think a fresh one every season or every 1500-2000 miles is the only maintenance needed.