Take a moment, look at the high end tab right now in party finder.
All those groups are people grinding away, learning to do one of the current EX fights.
It'll take them weeks to learn it well.

... and then most of them will forget it once they've not done it after a while.

You want a roulette that would require weeks of effort to learn every ex that ever was.. so, say a year of hard work to get every fight down, and not forget any of them, because you might end up in any of them.

Not only that, but you want 5 or 6 people with that level of skill to be qued up at the same time?

Look, I'm all for letting mentor roulette be accessible to more people, but inflicting that on the general population seems unkind at best.

If the rewards are somehow high enough to make grinding for a year to learn everything, and being the kind of god player who can actually remember it all, then greedy random people will drop themselves into the que, with no skill, but a LOT of hunger, and you'll actually drive the net knowledge of people qued for an ex DOWN.