The [ServerFault FAQ][1] states that answers from people affiliated with a product or service are allowed, but that the affiliation must be disclosed in the answer:

> **May I promote products or websites I am affiliated with here?**
> 
> Be careful, because the community frowns on overt self-promotion and
> tends to vote it down and flag it as spam. Post good, relevant
> answers, and if some (but not all) happen to be about your product or
> website, so be it. However, you must disclose your affiliation in your
> answers.
> 
> If a huge percentage of your posts include a mention of your product
> or website, you're probably here for the wrong reasons. Our
> advertising rates are quite reasonable; contact our ad sales team for
> details. We also offer free community promotion ads for open source
> projects and non-profit organizations.

Besides, the best answers often come from the vendor, since they have the highest familiarity with the product. As long as it's not spammy and overly promotional, I don't see why we shouldn't allow vendors to answer questions relating to their products. 

  [1]: http://serverfault.com/faq#promotion