Wouldn't that completely ruin the game? You'd basically have point vs point and not unique units vs unique units. What I mean is that by your suggestions 500pts of skinks should have an equal chance of killing 500pts of chaos warriors and vice versa. The game would basically become much less random because you'd always perfectly well know your odds. You'd know that regardless of what you do your unit should stack up to your opponent.
Besides not all army are equal to begin with. We don't have proper artillery. Empire don't have *good* characters. Chaos barely have any shooting and their magic is mediocre-good.
Why would chaos every be interesting to play with if you know that whatever you do - whatever you bring - your units will be equal to your opponents. You could bring 500pts of chariots, 500pts of chaos hounds, marauders, warriors and what not and the end result would always be the same. You'd have an equal chance agaisnt anything else worth 500pts regardless of what you'd actually bring.
Warriors of Chaos would be the most boring faction to play and yes you'd have all units be equally good, but also become equally boring in a sense. Why would anyone differentiate between Warriors from core and the chosen warriors from special? Why would you bother with Templeguards vs saurus warriors (except for the slann of course). How about Gors vs Bestigors? Elven spearmen vs white lions.
One could do what you suggest, but then again couldn't we just settle with one set of stats and rules for each categori of unit. What I mean is that a saurus warrior, spearman, dwarf warrior etc all had exactly the same stats and the only reason you'd chose something different would be purely for the fluff.
I might be taking your suggestion very literal, but that was what you wrote when you said the following:
The goal should be that 500 points of __________ (anything) is the rough equal of 500 points of ___________ (anything else).
And sure I can see how it would allow for perfectly equal units, I'm just failing to see the fun in your suggestion, but maybe there's something I'm missing.