The only thing that concerns me about suppliers on forums is when they are so narrowly focused on advancing their own agenda that they continually want to promote their own business or make biased self-serving statements within the general discourse. That is, posts that are not demonstrably to the benefit of the members of the forum.
If they are explicitly informing the board about wizz bang products they carry or specials they might be running or a new load of fresh widgets, that's all fine and above board, provided they do so without gilding the lilley; discussion boards are, afterall, about the exchange of information first and foremost.
Sponsorship can probably come in degrees, ranging from 1) small anonymous donations to 2) a page of clickable sponsor names to 3) non-exclusive prominent logos to 4) exclusive site naming rights. Obviously the value of each option is different, and the site owner(s) need to be aware of the degree of control/independence that they cede to the sponsor in each case. Preferably they should cede no control at all.
The model I think that works best is the one that we seem to be moving towards, but has not been explicitly stated. Non-exclusive sponsorship, with sponsor logos appearing in header space, sponsors being bound by posting guidelines like the rest of us, and no supplier--sponsor or not--being immune from critical mention should someone have a genuine grievance. What sponsors should get for their money is the right to display their logo. That's it. Any kudos they get from helpful and informative activity on the board is obviously going to flow through as a positive to their reputation and their business. If the primary purpose is to help and inform, then more power to them. If the primary purpose is to pursuade and mislead, it will be seen through and hit for six.
In other words, as long as sponsorship or advertising does not interfere with the free flow of information without fear or favor, then let it be. Come one, come all. So far I haven't seen anything to be concerned about, but Ray's points were worth raising and this is probably an important conversation for the forum to have.
My 4c
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