I run mentor roulette every single day. I almost always leave with at least one comm - sometimes (like this morning) with all of them.

Don't try to conflate a single data point to a global issue.

Also - not all mentors are going to mentor the same way - which means, not exactly the way you expect them to.

P.s. the mentor tome bonus is trivial to the amount of time spent. If I want to farm tomes quickly and efficiently I'll put a Ravana or Bismarck EX farm together, or just face roll Palace of the Dead for a few hours.