I think we should be encouraging vendors to participate on the site as much as possible. The major caveat is that it has to be an actual answer like you would expect from anyone else. For example it should not:

 - Just tell them to contact support (unless maybe it gets to the point of being a bug, in which case they should include the resolution later)
 - Be non-technical, this is a technical site. This means no marketing

It should:

 - Be a direct and technical answer to the question
 - Come from a developer or high tier support usually

A good example of this is [Willy Tarreau][1], the main developer for HAProxy who regularly answers questions on this site and Stack Overflow (even though that is a free open source load balancer).

On the vendors side, if they do it right, in my opinion it is **better than any advertising they could possibly buy from Stack Exchange**. Seeing the engineers behind a product reach out beyond their normal support methods says to me that the company is passionate about making something awesome. That puts them at the top of my list if I ever need a solution that they offer.


  [1]: https://serverfault.com/users/57371/willy-tarreau