I'm often asked this question from sellers. My response is the same every time. No, Open Houses are not a waste of time.
It is not possible to predict with certainty where your buyer will come from and so I am a firm believer in turning up the volume on all possible channels, and that includes Open Houses.
Also, I find that Open Houses are more - not less - popular with today's buyers who appreciate the anonymity and flexiblity they provide. Buyers move a long way along the sale continuum using online search and information resources. Open House visits are a good leading indicator that a buyer is progressing and ready to move beyond their online experience to actually touring and considering homes. Just as with their searching/browsing, they often prefer to take that step on their own terms. Open Houses allow them to do so.
Even buyers that are working with me actively and exclusively on their search enjoy attending Open Houses to supplement the time we spend one on one. If the mood catches them on an Open House day they can do a little window shopping on their schedule and at their pace.
So while it is true that if a buyer has a real and urgent interest in a property he/she will likely reach out to an agent to make an appointment, why not leverage the opportunity Open Houses present?
If you hire me to sell your home, then I consider it my job to maximize the exposure of your home to potential buyers. Open Houses are one way to do just that. Assuming the seller is agreeable, I recommend Open Houses particularly when first listed (and after a price adjustment) and every other week thereafter until the house sells.