I find disconnecting to get out of a run to be dumb, mentor or no. The problem is a disconnect penalty doesn't matter, since the people doing that intend on just walking away and making a sandwich while the cooldown ticks away. Even if the cooldown were applied when people log in, 30 minutes is enough time to settle into the game before you'd want to queue anyway.
It seems more courteous to bail if you've made up your mind to not run something than make the party guess whether someone's coming back, or do nothing in the 5 minutes before they can start searching for a replacement.
If the cooldown were applied the next time the player logs in, that would make a lot more sense as a deterrent.
Square's never going to make people run things they don't want to, though; the best they can do is make content people aren't willing to eat a penalty to get away from. This is true no matter what the penalty is, because if it's particularly onerous they'll just not bother logging back in.
Like I've said before, the problem with EX trials in mentor roulette (along with MSQ in mentor roulette) is the penalty for leaving is less than the penalty for staying. While it's possible to increase the penalty for leaving, there's no way to do so without also increasing the penalty for people that need to withdraw from a dungeon for more common means-- such as not wanting to spend time with the people you're grouped with due to harassment, or maybe an emergency comes up and you no longer have the time to run the dungeon. Blocking (or threatening to action) mentors from mentor roulette is throwing away an extremely valuable tool Square uses to lessen queue times as well, so they're not going to make it any more exclusive than it already is. And yes, mentor roulette is far more exclusive than mentor.
Furthermore, the people that have completed all of the content mentor roulette requires are better spent on other (non-optional) content. The tank pulled into an EX trial or MSQ dungeon from mentor roulette could've been provided to someone wanting to run Castrum Abania or Lakshmi, both of which have nasty first-run queues because people typically use the roulettes to level alts.
While it's true you can to be kicked, the party is under no obligation to grant that request. Just going straight up AFK is inconsiderate to your party, as well. Being able to withdraw -- provided you're willing to pay enough of a penalty to make it something you wouldn't just repeatedly do -- is your ultimate personal failsafe. And yeah, it's actually pretty considerate compared to the alternatives.
The fact that ex trials filled by mentor roulette often collapse is incidental -- the advice is the same as it's always been: don't try to do endgame (be it 50, 60, or 70) content through duty finder. If you do, there's a much higher chance than normal the party will collapse. As a mentor that occasionally spams the roulette because he's bored, who is so far away from a mount he just doesn't care about it, that's my advice.