I was doing the Mentor roulette on my DPS and as always when I am being a Mentor, I:

1. Explain the fights
2. Give tips on skills if needed.
3. Make sure the encounter is fun for the newcomers.
4. All the other stuff that is part of being a mentor.

I do not do them for the commendation, but I noticed that whenever I do the Mentor Roulette as a DPS, I get 0 amount of commendation. I do the same thing as a Tank or a Healer and I am being showered with commendations. Your suggestion would make it very hard for those with less popular roles to get rewarded with their daily tomes.