FWIW, I think throwing out an idea on HN is not a terrible idea. The feedback and useful suggestions probably outweigh the risk, especially if you are already working on building the business (i.e. past the 'idea' stage).

I suspect it's a case of "each one on it's own merits". Some ideas would see benefit in being aired publicly. Others less so.

If you dont have people you chat to one-to-one (or in small groups) about your ideas then I can definitely see the benefit, yes.