I'm going to say this... I almost always give a commendation to someone in every roulette I run. There are times I don't because people jet out as soon as possible, or I don't give one because I just didn't think the rest of my team was all that competent, but those are the exception.

I will say, though, there is one time when I don't give them, because the way this game works can actively discourage me from giving them out, and that's if I'm doing something on a level 80 job but am one commendation away from the weekly challenge log and the XP bonus it gives out, I feel like that bonus is wasted if I complete the bonus on a max level job. So if anything I would say that in my case, if the weekly challenge logs were something you had to claim while on a specific job from the log menu, I would feel more inclined to give more commendations.