I was holding an open house once and an agent popped in, toured real quick and then asked me why I was holding an open house.  She seemed impatient, said it was a waste of time, so I quickly responded about the way I see it-a way of meeting people who might be clients, prospecting, a way to learn about the surrounding community, talk the home up and meet-greet everyone and a way to get some work done while alone in the house.  She left quickly.  I held two more open house for this home before losing the listing without a sale.  A few weeks later I met her again at a social function, but she had a big smile on her face, came over and said that she finally held her own open house and sold the home to the third family in the door.  To me, you never know whom you'll met or who might buy.

If the client is ok for holding their home open to the public, I'll do it.  I consider open houses part of the work required to be an agent if the client wants it.