Ah, you didn't say that; I didn't mean to imply that.
The entire operating system (gas system, recoil system, BCG weight, etc.) was designed with the buffer weight of 5.4oz because it worked.
Somewhere along the way, buffers got lighter and springs changed, and not necessarily for the better. Do they work? Yeah, they'll function. Are they ideal? Usually not. I think that the primary changes were made based on budget - looking at the DPMS LR beginning. Would you buy the parts from Armalite, or make your own, and make them different so you don't get your pans sued off? "Sure, grab that AR15 receiver extension and let's make that thing work..."
The specialty springs are the icing on the cake (Tubb flat wire, Sprinco Orange, Wolff XP Springs, and I'm sure more). Then again, even using one of those is "fixing the system" when it wasn't ideal, no?
I wish he came around more - he's a goldmine of information. And a good guy.