On a typical Expert or Trial Roulette I usually get 1-2 commendations on my dps, 2-3 if on tank or healer, so I'm not seeing any tremendous difference. That said, basing whether someone has done a good job on commendations received is ridiculous. Plenty of very good runs end with no one bothering to comm anyone—most bad runs even more so. You'd be more likely to get comms in a run that tries everyone's patience from picking a fight with another angry player or having one picked against you, and just being the guy they less disliked. Save a trial through instruction, you may get zero comms out of spite. Save it through carrying them in a way that's friendly, and obvious yet somehow not blatant, and you get comms.

The main reason I don't like being a mentor through mentor roulette is that I can't check at any later time to see if any of my more general advice was integrated or that the player's performance or enjoyment of his or her playstyle improved, and as such feel pressured to stick to just the most concrete or dungeon specific elements, which basic advice some 15 levels ago would have made obvious anyways and tend to sound more nit-picky, trivial, and generally based more around memorization than improvement.