I did for my Advanced Wreck Diver course, but never since.
For me it is just risk weighting. The odds of needing it are very small. If I did ever need it, I am sure I would sorely regret it, but I am pretty certain I could get out of a jam without it. I think the risk is small enough that I won't bulk out my gear by making room to carry a spare mask. If I dived in a (bulky) drysuit, I might take a different view, but I keep my gear pretty streamlined.
For me it is just risk weighting. The odds of needing it are very small. If I did ever need it, I am sure I would sorely regret it, but I am pretty certain I could get out of a jam without it. I think the risk is small enough that I won't bulk out my gear by making room to carry a spare mask. If I dived in a (bulky) drysuit, I might take a different view, but I keep my gear pretty streamlined.