Sometimes the posts are blatant ads that don't really answer the question, or they add nothing that was not already answered. But sometimes they do answer the question and provide value, while also subtly pointing to a product. I think the latter is acceptable, and I do it occasionally myself. (Example: https://www.quora.com/What-kinds-of-lights-are-best-to-comba...)
Consider this: founders have the strongest conviction for solving a given problem. They often created a product because they saw a gap in the market, and they tend to be domain experts. So, they are highly qualified to answer questions within their area of expertise.
If a post provides value and just so happens to offer a relevant solution with financial strings attached, I have absolutely no problem with it. I might even end up purchasing it, because by searching or asking questions on Quora, I am looking for a solution myself.