I try to only give advice based on personal experience. But in general, I think it's better to stay out of conversations unless both parties are floundering, OR, it directly concerns you.
If people are happy being sold a line, let them be. particularly if they do nothing to educate themselves about a subject.

I was in an electronic department store and a customer asked and assistant about wireless computing. The assistant clearly didn't know anything about it except price and was pushing the expensive (802.11g) model. I was right there and just asked the lady customer what she wanted to do with wireless. All she wanted to do was surf the net from her laptop. I explained the difference other than price in the two products being offered and justified why she could get the cheaper one, if that was all she wanted it for. I left the decision up to her.
I don't know what choice she made but at least she was informed.