Keep in mind, as well, DPS have more than just the positioning and timing of Limit Break to worry about - we have our buffs to worry about, too.

If I see that Limit Break isn't quite up, I'm going to go into an agressive burst-DPS cycle, most likely popping my best skills (Blood For Blood, Raging Strikes, Internal Release, Etc...) and start applying the bulk of my spike damage. It takes me about 8 GCD's to go through a full cycle, and if Limit Break comes up in the middle of that, it would be a net-loss for me to blow it at that point.

Furthermore, most the time, I want to apply a DoT before limit breaking. Why? Because it can continue to DPS while I'm limit breaking. It's an optimization thing.

I'll be fair and say that there's no way you can know that a DPS is doing any of this, and when one sees there's a limit break bar at low % HP, all they're thinking is 'OMG WE CAN WIIIIIIIIIIIIIIN!!!!!!', but keep in mind, not all of us DPS are terrible. Some of us take our jobs seriously.