So what you're saying is if the variation in one brand of motor is that big, lets throw three more into the mix and make that bell curve even wider? So you've got to buy 5 lottery tickets to four different lotteries to try to get your best chances of winning. And then you've got to buy 5 rotors for each brand, but wait some companies have three different rotor styles, so now its 15 per brand. Gotta get those ceramic bearings too, and different sensor board date codes, and the list goes on and on.
Not all that is true, and even the parts that are true are stretched to their limit. The idea that you can buy a motor, throw it in the car and be competitive is suddenly regarded as a fairy tale(whether it should be or not).
So as John said, even if all Spec racing does is lock you into buying one particular motor (or battery, or tires, etc), it is at least limiting the permutation a racer thinks he has to go through in order to be competitive. If a guy has the $1000 to buy all the stuff I listed (tongue-in-cheek) in the first paragraph, he'll still spend that money in a spec class (kind of like Jason said). But by narrowing the bell curve by specifying one component, you at least have a fighting chance statistically at picking the magic bullet out of the case.
That's how brushless racing took off, the idea you could grab a motor and go. The shift away from that is hurting all forms of power limited racing (on-road, dirt and carpet oval).
Just some thoughts. I know others may not feel the same way, and that's precisely why we have Open classes as well as Spec classes.