When (and if) I run a flashpoint, I will run ahead and I will pull things. Why? Because I tend to have more health than the current tank as a DPS. I kill things fast enough that tanking is not necessary. I typically run with a healer from my guild that is capable of 2manning it.
Now, why would I sit around and wait for a tank to take his time and pull everything, when I could just do it for him and finish the place in a fraction of the time?
If you are running a 50 HM that's true, so I may give you a pass here, but again why are you running these to begin with. If you are running a 55 HM as dps, regardless of your gear, and you start tanking... I will 100% kick you, especially if I am the healer. And my healer is in mostly 72s.
And BTW, your efficiency as a tank has nothing to do with your HP. Mitigation is what counts. 25K HP tank is much stronger than 35K dps in terms of mitigating damage.