Each time I queue for mentor roulette I'm actually hoping to get an EX-primal because I feel my time might be spend best there. I've cleared quite a few different EX-trials this way too.

If theres something that should be removed from mentor roulette its guildhests... they dont actually require a mentor. EX-primals are the content that really does - and you want to remove that because you're lazy and want an easy ride...