It seems to be a "best practice", but I see no reason for the extra expense other than it may be a bit tougher than regular flex if you're moving it around a lot. The only good reason anyone could tell me why it was used was because all the HVAC uses chilled water and water leaks happen. Some of our areas even have under floor water detector sensors. Don't think one has eveer gone off.
A wet area is not one that could getwet occasionally. To me, use wire with a W in it and regular flex conduit.