All about the marketing. Instead of saying it's to make your job easier - because really, it's as easy to spam Cure as it is to spam Stone - tell them it makes the run faster when you are DPSing instead of healing. Tank and healer can do around 75% of the DPS than an actual damage dealer does (100%+ for a SCH when damage dealers aren't AoEing) so the difference between you casting heals or casting damage spells is about a 25% increase in total group DPS, which means a 20% reduction in time spent clearing trash.
While I fully understand your point, I see the extrapolation of this view as the very poison that has DoT'd all MMOs. It is all about speeding everything up to get to some arbitrary finish line that doesn't/shouldn't exist in an MMO. Console single player games, yeah... I can see that. But when we start boiling everything down to the quickest way to get finished, and then in the same breath scream that the game has no end game and no longevity because we the players have figured out how to sidestep or distill the content down to the fastest avenue possible to get to this mythical trophy of level cap/end game... well that is just foolish.
Very few players now actually take a look around at the environments and the subtle mechanics of dungeons and appreciate them for just being there. We see them as obstacles to overcome as fast as possible and when this is not achieved in a manner we come to expect, from other games or a prior run(s), we turn on each other because we see this as wasting our time. Yes, you may have run it many times before and already know some routines, but never stop and think about the party you are doing it with and what their experience is with it (speaking for DF parties since you can do these with a set of friends however you wish). So low and behold, we have a time out measure in place with weekly caps placed on rewards and we cry about that because it impedes us from 'finishing' our goal, to win the game - though that cannot really be reached in an MMO, so we made up one by having BiS and this notion of farm status for high level content (meaning we have boiled the content down to the fastest way to finish the run) - and that is what we cry about not being able to reach fast enough. But then what? What happens after you wring out all the runs and get all the stuff?
It really does pay off in the end to find a niche group of like-minded people and stick with them because the rest of the community will eventually cannibalize itself until the next game comes out with a level curve to be flattened.