The ServerFault FAQ 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
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.