Usually because when I tank there is a noticeable difference when any DPSer drops below 1200. The fights take longer, the healer gets low on mana before the fight is done and overall you have to pay more attention to keep from dying/wiping. Better DPS clearly allows less time due to quicker fights and more breathing room in case something goes awry.
The standard prep for most heroics is having everything with a lvl 78 - 80 blue or better (either quest, BoE drop, or dungeon drop). It does involve hitting up the Final Three (Occulus, UP, HoL) to get the gear necessary. With that and a proper spec and knowing how to play your class should be enough to do heroics well enough to not make people question your ability.
Not to mention it's all good until the first boss. Once you down him/her, you're saved. Which ultimately means you are "stuck" with that present group. So you have to pay attention at the start and look for any potential trouble spots before you get to the first boss so you can bail and save it for a better group.