From what I seen people use target marks for different situations depending on what there doing pvp, ops raid or just group. I am a big pvp'er and mostly puggin it. I use target marks all the time. But the only thing I target mark is the healers, as they r the ones that should be killed first. I will leave a 1/4 health player I'm fighting if I see a target mark healer on my screen, I'm going after the target marked. It is annoying when others r beating on a tank, wile the healer stands next to the tank with a big target mark above there head and just sit back and heal, heal, heal.
I don't look at classes or names just the healing cast bar from most healer Is how I find my target to mark. So when others mark non-healer classes its so annoying. I'm beating on a target mark and there life is not going down then I realize that some n00b target marked a PT or Jugg. I always take the target mark off non-healer classes. Just my take on target marks for pvp.
On a side note I use the following target marks in order. Cross hairs first as its big and gold. 2nd would be star cause again little smaller then the cross hairs but main reason it's bright gold and can see well. These two alway go on sages/sorcs as they r my first targets. 3ed would be the shield, cause again big and blue. This going on op/sound, merc/mando. Next would be the coge, harder to see size wise and color compared to the others I said. From there it's random if a game has that many healers (witch I seen). The one that I don't use at all is the flame. Reason being at least for me is that it's red. Enemy's names all come up red. So the flame blends in well with there name and is harder to keep track off. Saber and lighting bolt r up in the air as the colors r easy to track the icons r skinnier than the others so again little harder to track.