The first time you do a dungeon, it is fun. It's probably still fun the second or third time. After that, the fun starts to wear thin, and by the hundredth time, you're not there to have fun - you're there to get it done and move on. That's not to say it isn't fun (though many WOULD say that) - it's just that the source of the fun has changed from "I want to explore this new environment" to "I want to progress my character". In any dungeon, you eventually reach the point where you value efficiency over admiring the scenery, and that's where speed runs come into play.

DPS aoe attacks have the greatest damage potential of anything a DPS can do. The more mobs you pull, the more damage your DPS can do using these attacks. When striving for efficiency, the best thing a tank can do is pull as many mobs as they can and still survive and maintain enmity.