They are either misinformed and still think it's their HP that affects their healing of party members (fairly rare, but you never know)...
Or, as I alluded to in my first post and as Phive said, they are trying to survive at a place like Grims or DTs a few levels early, so they can turn MG off and save money. Which is still a very valid reason to get HP equips, as long as you don't sacrifice your damage if you're attacking. >_>;