That increases your DPS of course, but it does not do anything to beat the encounter. And there are several cases where it depends on your raids tactics AND if you are playing 10 or 25men. I'm in a 10men raid - you seem to be in a 25men guild (at least thats what wowprogress states).
Second part of your post is correct - no one doubts that and it was said several times. I just protested against the people who highly exaggerate the advance.
If you want to say that destro is not even good on more than 4-5 encounters, then you are discrediting yourself. A simple look at raidbots would show you, that on average destro is as strong if not even stronger than affliction. In 25men & 10men as well as on average and max ranked. So what are you trying to tell us?