What I'm suggesting is that that's the irrational part. They should be thinking about this decision as though it's an offer of a million dollars to move to Austin, TX (because it is).
If it's paid off, it's perhaps fine. But many people owe mortgages on those houses. They will be in for a world of hurt if house prices drop. I.e. The exact scenario we are discussing here.
Agree that holding mortgages that expose you to negative equity risk is a big gamble. Like stocks (real stocks, not options or non-listed equity) it's actually a good gamble if you will be able to cover a 5 year price dip. If you are likely to have to do a distressed sale at a stress point you are setting up a big loss - smart to take the risk if you are in your early 20's (the upside is great, you'll be ok if you bust) but dumb if you are >45.