It may not add much to the discussion, but it did say the mentor roulette is specifically designed so it is easier to join your student in a dungeon. So my guess is you need one of every role, so that the dungeon can be run with the proper party makeup (because if a tank is teaching a tank, no dungeon under level 50 will have room for both of them) so you'll need to take the role that they aren't, and on top of that you'll be able to give them advice in the dungeon. The level requirement is just ensuring that you have had direct access to the core mechanics of each role, you should be able to tell a baby CNJ "Hey don't worry, your mp management becomes easier once you get Shroud of Saints and then later you'll get Assize," WHILE you are DPSing or tanking in the dungeon they need help in. The game is ensuring that you can become flexible enough to accommodate a new person undertaking any role, and not make them wait on you while you form a party so you can both go in and tank or heal. The comms I /think/ are just a guard to make sure you're not a complete jerk (though the system is flawed, admittedly) and the dungeon requirement (while high) is making sure you didn't just power level in fates, and that you could run the content blindfolded so that any questions a newbie may have, you can answer.

The system isn't designed to cater to the mentors. It is designed to cater to the students, and making sure they are getting help from qualified people who know what they are doing. Not a perfect system, but better than just judging everything on raid content or play time.