Unless you are a savant (computing how much threat you are generating by watching the combat text) or your tank ludicrously out gears you, then as a DPS class you can't reach your peak without a threat meter. Simply not possible. Im talking about riding a few crits from pulling aggro, you can't push your DPS like that without a threat meter.

Sure you can do easy content or farm raids without a threat meter, but if you are progressing content then you not functioning at 100% efficiency is most likely costing other people time and gold. Before BC most raids where designed before threat meters and so didn't have enrage timers, and DPS could take it easy, you could take two hours to kill a boss as long as you could keep healing that long. But Blizzard realised people where using threat meters, and so started to add fights which required you to push DPS right up to the cap (enrage timers, and fights with lots of aggro dumps).

Raid mods are tools, idiots will still die with the best tools in the world. But good players will reach heights that they could not without those tools.