Originally Posted by
Charge me Doctor
If you put on heirlooms you basically say that "i want faster leveling experience and don't want gear anymore", dungeons always were for gear, you won't be able to properly level when undergeared (thanks to cata quest rewards give you adequate gear most of the time), when you powerlevel your gear gets outdated really fast, so you have to run through couple of dungeons to grab at least a good weapon (in case of non-casters) for your current level, so you won't spend 20 seconds beating one mob.
Dunno about Draenor, i'll gladly skip this content because of garrison and how questing was paced there. In legion i didn't noticed that (frankly, i've only played in 7.3.5 from 108-110) and because of artifact weapon dungeons feel very trivial. But legion gives you a lot of quests that make you go into dungeons, so there is that for incentive to do them.