I have to be missing something. "Layers" isn't the right term, but I understand what the idea is.
In every edition of D & D you could pick and choose what supplements and rules you wanted. In 1E you could or could not use the Unearthed Arcana, Dungeoneers Guide, etc.; in 2E you could or could not use any of the additional books; in 3E you could or could not use splats, 3pp, etc. (I don't do 4E.) And that has nothing to do with houserules and homebrews - which trancend all editions.
So, why is this seen as so novel?
There are two common complaints against splatbooks and other optional rulesets:
1) Power creep.
2) Increased complexity.
If you put a "price" on their use, options can be unabashedly more powerful (such as e.g. Gestalt rules), without complaints about power creep - assuming that the price is obviously correct, of course...

And second, since there is a price, players will only ask for those options they *really* want - no unnecessary complexity. People will probably want to buy (or subscribe to) the splats, just to have the option available - but they will not have the large complexity of "everything is core", since just a fraction of the options will be in play at the same time.