Open houses are excellent for generating for future business. But I welcome any statistics or facts that can link sales of specific properties as a direct result of specific open houses.
Our office did have this happen at least once. The agent was putting up a sign in front of the house the night before. The neighbor came out and said his friend was visiting and could he look at the house THEN, before the open house. The agent said "sure" and guess what, the friend made an offer (and eventually purchased the house) because he didn't want to run the risk of losing it.
Another couple in our office had an open house and people saw it on the website, traveled 2+ hours specifically to see that house. Unfortunately, the house was too small for them BUT they made an offer on another house in the community. That wasn't even "future" sales, that was a sale that weekend resulting from that open house, even if it wasn't THAT house that sold.
Open Houses, like anything else, will work better for some houses than for others, work better in some markets than others, on some days better than others. Giving a blanket statement "They're a waste of time" is foolish. Not everything works in every market - that's true. Farming postcards can be a waste of time for a lot of agents. But that doesn't mean it's always a waste of time.
Know your market, know what works, and work it well.