People quickly forget that one of the things that has kept this game going for so long are time sinks. The longer it takes to get something the done, the longer you're playing (and paying) to accomplish your goal. This includes NMs. While I don't think things should be as extreme as HNMs, I do believe that you shouldn't be able to just mass kill fodder mobs so you can spam an NM until you get your shiny either. Not every single item should just drop into the hands of every player that wants it. Some items are meant to be at least somewhat rare.
Easy to complete content isn't good content. The faster you complete it, the faster you get bored, and the faster you quit/wait for new content that you can just steamroll to get back to waiting for something new.
Most of this I answered in the rest of my post, but I'll also add that it adds competition, which leads to a bit more of a sense of accomplishment when you finally get the item/items you're working towards. I'd find having to camp an NM for a reasonable amount of time far more fun than slaughtering EP mobs to get a pop to camp a set of ???.

