This is just my own private theory, but in actuality Paizo has made some great effort to provide 4E content in their modules. Not 4E stat blocks, of course (these are pretty easy to pick up if you got the 4E MM1 and MM2), but modules that contain more than the occasional nod to 4E.
Just to mention two examples - and I recommend everyone to check on them in person when next leafing through these products in their FLGS:
"The Armageddon Echo" has people visit an Elven Town in the
Shadowfell, and "End of Eternity" has the party visit the
Elemental Chaos replete with Primordials (renamed "proteans") - the Pathfinder issue even dedicates a whole ecology article to them.*
That's just two individual instances, but for me these products fill a nichè. I love the 4E MMs and appreciate the ease of plugging 4E monsters into older modules. But 4E modules by WotC themselves are, as Primal put it, not that in terms of plot intricacy. For me, any 4E module you buy in the shop serves the same function as
Dungeon Delve - you get some pretty exciting encounters which you can weave into an adventure of your own making. Or - and here I get back to Paizo - you could combine them with someone else's story-rich offering.**
The main issue, of course, is why you would want new or contemporary ("story-rich") modules at all when the past can serve. Well, that's true to be sure but, as I said, it IS refreshing to see a company put some good effort into making the new cosmology tick and bring home their
exotic nature in a way that WotC' own products (MotP, P1, P2) continuously fall short of doing.
So here's my overall conclusion of the points I raised.
To contribute highly valuable stuff for a 4E DM, a 3PP company doesn't need the GSL. While the GSL will help a 3PP product to get recognition, beyond that it's simply hampering. A 3PP product could well be more rewarding to a 4E DM if it is complementory and not supplanting of 4E product; that is, if it doesn't even attempt to include or create new 4E mechanics when WotC is far better suited to provided these and there's already aplenty thereof. That was
my verdict on Kobold's Quarterly and is, in fact, my verdict for the 3PP product I've liked best in the last year. Leave the mechanics of exciting encounters and powers to WotC, and give us some story meat to put on these bones!
* There's a third instance, similar to these, but I leave it to others (familiar with the product in question, which I am not) to verify it: "Memory of Darkness" actually portrays a
Feywild environment.
** See, I don't get people who say "Why should I buy 4E
Kingdom of the Ghouls when I already got
Dungeon 70?". These are two completely different but strictly
complementory options, and putting them together on your table will create a terrific set of sessions.