I am not going to notice or care if a MNK has Greased Lightning up full time. However, I do care that they care about it, and if said MNK pulls a mob or boss to keep it up without communication to the group, they will hear a little something from me about it. I know you're just using this as an example, but generally speaking when I run content as a healer, I am not paying attention the damage DPS players deal. I am paying attention to the damage they take. The only time I ever question the amount of damage they are doing is if phases are being pushed too fast, or too slow, leading to wipes.
Casual does matter because things like optimal CD rotations for tanks and healer DPS are not required to clear it. Of course I want everyone I group with to put in their best effort, and to know the do's and don'ts of the instance. Tanks and healers optimize their play by choosing the best action according to what is going on in the field at the current moment, and if nothing requires special attention, they contribute to DPS as best as they can. DPS optimize by not only pushing their numbers as high as possible, but also making things easier for their healers and tanks by handling the mechanics such as interactions that would otherwise distract the healer and/or tank, paying attention to not steal agro, and generally just not screw up by getting themselves killed.
Casual content is designed in a way that not all of these things need to be present in order to clear it. I get what you're saying, and I agree that just because it is casual doesn't mean it is ok to play poorly. However, what I am saying is that because it IS casual, you have to expect that you're going to get a player here and there who is just going to do what they do without consideration to others. In these cases it is just best to get in and out of there. Raising issues only adds even more time to the run, and if it's something trivial like a DPS who does not AoE, a healer who does not DPS and so forth, then it is actually a disservice to not only yourself, but the other members of the group who do care.