I touched on this in another thread, but I think it's worth discussing separately.
When 3E was released and the OGL and D20 STL were introduced, it was, without question, a huge boon for the gaming industry. WotC was willingly and freely giving other companies access to the rules and even lots of the IP of Dungeons & Dragons, making it possible for the first time for third parties to create and sell products that were explicitly derived from and/or connected to the most popular RPG of all time. Not only did this help many new companies get started and make profits, it also helped support 3E and ensure that the new edition took hold and flourished.
The ease with which the OGL/D20 STL could be used and the (seemingly) easy money of the early days led to a glut of bad product that damaged the D20 brand to an extent (how much is a matter of debate). But, even with that, there's no denying that the OGL had been a success, an amazing gift that allowed previously non-existent companies to arise and prosper. Though many D20 companies died, not all of them did and there are at least a few -- Paizo, for example -- still existing who owe their very existence to the Open Gaming movement.
This was a blessing.
But then came v.3.5, which was unexpected and unwanted and, from what I have gathered, it was this revision that mortally wounded an already-hurting D20 market. Many companies dropped D20 support as a result; many more died entirely -- all because WotC made a decision that was good for its business and without concern for how it'd affect third party publishers (I should make it clear I'm not attributing malice or even negligence to WotC. I am simply saying that its rational business decision was almost certainly made without taking D20 publishers into account and, frankly, there's no reason why it should have been otherwise).
Therein lies a curse. WotC will do what WotC will do for their own reasons and no one else's. It seems to me that, while the OGL gives unprecedented access to a piece (albeit a small one) of the D&D pie, it comes with a steep price, namely being at the mercy of WotC's future decisions, whatever they may be. There is no way for a third party publisher to plan for the springing of a new revision, edition, or other major change on the gaming public. That means that, in the event such a thing occurs, they're left scrambling. Just look at Paizo now and you'll see a good example of what I mean. The OGL binds a third party's fate to that of WotC's unpredictable future decisions and that limits the ability of a third party to plan for its own future.
The conclusion I draw from this is that, unless a company wishes to forever remain beholden to WotC's decisions, it is in their best interests to use the OGL to find their own path and forge their own destiny. Both Green Ronin and Mongoose provide examples of companies that have created their own games and IPs from the D20 base and run with them, expanding and developing the original systems (and even developing new, non-D20 systems), in the process making themselves "edition proof." You see no fretting or angst at companies like this about the advent of 4E. They can make their decisions based on their own assessment of the market and their customers, not WotC's.
I'd like to see Paizo survive and prosper. Whether they go 4E or not, I'd like to encourage them to think of the currently unimaginable future when 5E or whatever is on the horizon. Do they want to be in the same position they are today, worrying about which way to jump and which fans they may or may not lose by doing so? I wouldn't think so, which is why I hope they'll see the OGL for what it really is: a gift with a large hidden price tag.