I don't have any problem with developers using our site for 'dedicated' support, the problems I do have are:
(Synergy) developers do not monitor this website regularly. This sounds to me like take, take, take, but never give anything back. Furthermore, I'd expect the developer to behave like any other user and also answer other questions or else limit himself to providing support to questions about his own product. This should not be an excuse to spam your product on every related question.
The type of questions should be Q&A questions. Jon Skeet had a nice example here
- How do I? -- StackOverflow!
- I got this error, why? -- StackOverflow!
- I got this error and I'm sure it's a bug -- file an issue!
- I have an idea/request -- file an issue!
- Why do you? -- the mailing list!
- When will you? -- the mailing list!
- You suck and I hate you -- contact us privately at email@example.com!
You're awesome -- aw shucks!
Basically: we want questions about you not knowing how to do something. All the rest are bug-reports, feature-requests or discussions and off-topic for any SE-site (AFAIK).
I disagree somewhat with Rebecca that using Stack Exchange for dedicated support is backwards, we have to start educating people that we provide answers to their questions. Developers pointing to us only emphasizes that. However, it should be clear what we're there for and I would almost demand there's a dedicated person who keeps track of the questions of their product. If they see something that doesn't belong on the Stack Exchange site, it's their duty to help us keep it clean.
I do believe that this could be a win-win situation and there should be no need for separate SE-sites if others already exist. It just takes more than a simple link to our sites to make it work.
Update in response to Robert's answer Note that the questions should be limited to performing tasks with the software, no bug-reports or feature-requests. While those can be obscure too, they will help any other user of that product.
As for the MSDN comparison: I think its unfair to point to Stack Overflow and its a chicken-egg problem. Yes, we want to get these questions organically, but that's assuming we're THE source for that topic on the internet and everybody out there knows what we're doing. Newflash: none of the other SE-sites has truly reached that status and even Super User doesn't even come close to the ubiquity of Stack Overflow.
So I would applaud any company who would want to collaborate with us, but that doesn't mean its free for all.