I can't agree more about those particular people contribuiting a lot. Everybody here contributes in thier own way, to their own ability.
I see it go the other way too, where sale threads ought to have the tech discussion moved to another area. Why? because many don't see the discussion when it occurs in the for sale section. Ideally, you have a for sale product thread, and you have a review from a customer thread added later for a long tech discussion.
I couldn't tell you where the line should be drawn. My personal FAQ is often who to buy a kit from. It would sure be a pain to have answer that every time in a pm to avoid "selling" in the ebike general section. Unfortunately I do get biased on my advice, since I tend to say, such and such worked for me. But I don't get free stuff to review from most of the vendors. If I have the cash, and the need for something, then I buy it and review it. But it's still not a representative sample of the whole market. I definitely have ended up with lots of 9 c motors, some free, some I just got because I like em.
Once in a while though, you will see a vendor get a bit windy about his latest product, nudge nudge wink wink. And I have seen the mods take a stand on that. This being ES, any exaggerations get the dope slap. So wherever a vendor posts, he better speak the truth. 8)