Well, one thing I've noticed is that some DPS are confident enough in their abilities that they continue pushing a DPS by themselves, despite the mark being on the healer. So, the damage is split between me (PLD) and one DPS on the healer while one of our DPS chases another one, trying to kill him. Now, I don't encourage this sort of reckless behavior by any means since:

A. It undermines my authority
B. It shows you're not a team player
C. It's reckless and can cost you a death (which in turn costs your team a loss)

However, I did witness one DRG in particular that did this sort of thing and kept getting kills in 1v1 fights with DPS, so I let him do his own thing since he was good at it. After seeing the results, I realized why... He was pulling anywhere between 175k to 200k every time he was on my team, lol. The only thing that really frustrated me tonight was I got into a match and marked a DPS with the offense buff to kill in the beginning, but our DPS' burst was so terrible, we didn't even bring the guy down to 50% of his health and that was with my maxed out Full Swing vulnerability debuff active. After realizing we weren't going to kill him, I switched to their WHM and wanted us to focus them instead so we could take them down and get our DPS kill since our damage wasn't higher than their healing, so the healer had to die for us to gain the high ground.

That mark stayed on their WHM from the beginning to the end of the match and never moved for one reason... Our DPS never killed them, lol. I popped her with my vulnerability debuff when it was ready, then one of our DPS decided that was the best time to switch to another DPS and try to kill him instead (at full health no less). If the marked target never dies, that just tells you it wouldn't have mattered who you marked in that fight. You just don't have the numbers to do anything with that team and that's just the worst thing to see.

Here you have to play out the rest of this charade for the next few minutes, knowing your DPS aren't strong enough to kill anybody, but you don't want to just give up... It's so frustrating sometimes, lol.