The recent release of the White Star RPG has started a discussion about settings and rules. Tenker weighs in with two posts, here and here, and there is some excellent comments attached to both. Even my friend Tim of Gothridge has some good thoughts on the matter.
What I would like to point out is that what came first in the world of tabletop RPGs were the campaigns, notably Blackmoor, Greyhawk, El Raja Key, Tekumel, etc. The rules came afterwards. From reading Playing at the World, Hawk & Moor, and other accounts back in the day, the focus was on one thinking of doing something interesting first, and then figuring out the rules to make that happen.
And based on my own experience in the late 70s and 80s, that pretty much the natural way of approaching it. Rules are important as they enable the whole thing to be a challenge rather than just some elaborate daydream or break apart on the "Bang you're dead, no I am not" issue.
The importance of DnD and other published rules is that they show what possible. My first encounter with the DnD rules ignited my imagination in a way no other game had to that point. Compared to the Avalon Hill and SPI games I owned, classic DnD was a uninteresting wargame. But it was perfect for exploring worlds that only existed in my imagination with the virtue of being able to do it in the time and budget of a hobby.
In my view White Star and many of the other OSR games are not innovative examples of game design. But where they excel is in exposing words of imagination in a way that makes people go "Yeah I can see how that done." and inspires them to realize their own worlds. Again within the time and budget of a hobby.
I will say that while I appreciate and respect settings like Tekumel, Spears at Dawn, and Arrows of Indra. They are not really my cup of tea. My preference is for bog standard fantasy world but with depth; Harnworld, Ars Magica, or the old Thieves' Guild from Gamelords. I think we only scratched the surface of the interesting adventurers that can be set in such worlds.
In short we need both and that it is all grist for the mill in coming up with adventures and campaigns for our players.