Wotc's motives are obviously something that is very speculative. We have really two options for how to judge it from the outside in (aside from corporate espionage

)
1) Make an assumption about their position and goals, and then draw conclusions (i.e. are they trying to kill off the 3pp or work with them). I know WOTC (and Scott) are saying they intend to work with the 3pp, but actions speak louder than words. If there was a movement within WOTC to rid us of the OGL, we would not hear about it publicly, heck, Scott might not even be privy to it.
2) Look at the facts and try to discern from them what WOTC's strategy and plans are.
So far, the actions are very negative towards the 3pp. Let's look at the facts:
1) The GSL terms shut down the OGL and give no safe harbor. A 3pp who comes into the GSL cannot take their product back to the OGL, and WOTC can at any time cut the 3pp out of the GSL without warning. Any legal matters that arise are all out of the 3pp pocket. Basically, anything and everything that could be wrong with the GSL from the 3pp perspective is wrong. WOTC in that agreement is a lion asking you to put your head in its mouth and trust it won't bite down. If at anytime the lion gets bored or hungry, it can bite. Would you do it?
2) Profit driven company's do not do something for nothing. WOTC has something to gain here. What is it though? Now, looking at the big picture, the OGL brought everyone to D&D. I had customers who stopped playing any other system because of the OGL. All of the company's basically hopped on board the d20 movement, and the few company's that didn't are looking foolish for not doing so (Palladium being the lead offender, Rifts d20 should have happened).
So, WOTC puts out the OGL and the D20 license. It makes WOTC look good for sharing D&D with everyone. As Ryan Dancy said, their surveys showed that all rpgs brought people to D&D, and that by unifying the d20 system with the OGL it would allow for roleplayers to easily share their rules and games. Now, alternatives to the d20 system are nearly extinct (except for White Wolf, Hero and Gurps for all intents and purposes). What did WOTC gain there? But that is not all..
Now, we have D&D4e. Like it or not, D&D 4e by its brand name and eventually a good library of support material, will be in control of the industry as D&D has always been. The GSL's key element is how it shuts down the OGL and ties the 3pp to the D&D player's handbook. You can't even print material from the core books (as Mongoose's preview of their cryomancer shows). Basically, the more core stuff you try to use, the more references "see the player's handbook" line afer line in your products you have. This makes for a product that is inconvinent for the customer, but convient for WOTC. This is also a sign of what WOTC wants and what they can gain. Basically, the company's publishing for 4e will drive all of their customers to 4e's player's handbook (and open the door to have to buy mostly WOTC D&D products). When WOTC releases 5e I garuantee there will not be a GSL. Also, I would not be surprised if after a period of time WOTC just closes the GSL. Which brings us to our next fact:
3) Their is no out clause in the GSL for 3pp, nor is their a terms of license continuancy. Now, I am not a lawyer. But I am a business owner who has worked with lawyers and contracts. When you draw a contract between two parties you want both parties interests met in the contract. If they are not or they are vague, then that is a bad contract. The GSL is allowing you to build a business on supporting D&D and building D&D's brand and WOTC doesn't have any contractual obligation to respect or protect that relationship. This is very bad. If I build a who series of products that boost D&D sales from my niche, and build my whole company around supporting 4e, then WOTC takes it from me, I stand alot to lose and have no legal recourse. I see posts here about well, WOTC is given you the rights to work with them, and its free. But the thing is, it is not free to work hard to build a business, build D&D even stronger for WOTC and then have WOTC snatch that away with a whim. That is dishonest. The 3pp have rights and deserve to have their contributions protected if WOTC wants to share the D&D sandbox, any other alternative is just a bad contract.
4) Then you mix in the downsizing, WOTC having issues getting the DDI to launch, Gleemax dying, and some major steps forward as well as backward that 4e made (though more forward than back in my opinion) and it is obvious alot of upheaval is occuring with the company. Hopefully it pans out for the better, I would not say it is a bad sign persay, though I think WOTC has a tendancy to bite off more than the company can chew and the leadership at the executive level doesn't understand the hobby game industry enough, while some of the visionaries within the company coming up with great ideas do not fully see the obstacles ahead.
So, what I see is a company with good intentions amognst its gamer employees who work for a corporate beast that wants 100% control of its interests and assest, that underestimates that hobby game companies, and thinks it can treat the game industry differently than the big business it deals with. I am sure Hasbro only takes parts of what WOTC does serious, and Hasbro's main interests is in building the brands in to mainstream elements. Hasbro would rather have a D&D mmo than a rpg for example, which perhaps the 4e path is the first steps in developing that.
The bottom line is what makes the most money for WOTC, while not hurting their public image. That is the key.
I have to cut this short due to time. Feel free to add or comment and I will try to complete my thoughts soon.