-edit-
I meant PVE.
They are the ones that have shown the biggest commitment to the game.
They help newbies thrive in a challenging environment.
I believe that mentorship should be a prerequisite for housing.
If this isn't possible, then I believe mentors should have houses for free. Anybody else has to pay the standard amount of gil for them.
If a house that a mentor wants is occupied, they can pay triple the cost for an instant eviction of said homeowner (assuming said homeowner is not a mentor).
At the moment, there are just not enough perks for being a mentor and helping this game to blossom into the amazing game that it has become. I feel that housing is a great way to incentivize mentors.
Craft mentors are excluded because that is damn easy to achieve. Any drooler can score craft mentor in about an hour.