The way I see it, mooks provide a bridge between conflict and descriptive type games.
That is, in a conflict game, it's all about chance and tactics.
In a descriptive game, it's all about style and entertaining roleplay.
In a conflict game, you rarely can do flashy and amusing actions, because likely it means you roll badly, slip off the chandelier and break your back.
In a descriptive game, you rarely get the thrill of potential loss and astounding success.
With a mook rule, you can cleanly switch between the two. Mooks allow people to play their tropes and have the freedom to do wacky, freewheeling stuff. Woohoo! Swing on the chandelier, baby! Use one mook to hit three others over the head. Etc.
But then you go up against the non-mooks! Switch to conflict, and the drama is highlit...
I personally think it's a brilliant design, as someone who likes both modes of play. If you find one mode interferes with the other, of course, you'll hate it.
Some people, on the other hand, find the rule irritating beyond belief. Claims have been made that the rule is tantamount to racism, by creating an underclass of non-people who don't count in the world like the PCs. The vehemence puzzles me, but if you are going to use the rule in your game, you might want to find out if it will twist any player's knickers.
One thing I like about D&D is that at a certain level, low level opponents sort of become mooks. Sure, d&d lacks some of the support for descriptive flexibility, but people can resort to it if they want. In any case, a 6th level party facing a bunch of 1st level orcs is probably going to sweep the floor with them.
And people who hate mook rules usually don't mind that sort of 'emergent' mookishness.
Oh, a last point... one thing about mooks that is more of a convenience issue is that it makes NPCs a lot easier to run. If you have 50 NPCs, it's a f-load easier to treat 'any hit drops them' than to detail out what each hit does or track 50 sets of damage. Man.
And hey, in D&D, conscripts in armies already have that rule. They get hit at all, they run.
