I can see why people might consider it as a very short-term move, but I would never do it.
I don't agree with the argument that in 20 years the mortgage value will seem so much less - we took out our first mortgage nearly 17 years ago, and while that amount is obviously 'less' now in the sense that house prices have risen and so have wages, it is still not an insignificant sum.
Looking back at our first mortgage, we could not afford now just to pay that off, no matter that inflation might have eroded the value of the original figure.
I do know people who are planning to pay off the mortgage by downsizing, but I still think that is a gamble. We might downsize when the children have left home, but to us that will be to give us a choice as to when we retire, where we live, what we do etc. Not a necessity because of a mortgage that needs paying off.
And I'm just to stingy to part with loads of interest over those 20+ years, it's really satisfying to see both the mortgage amount and the interest paid going down each year.