The answer to your question is very "local." Here in Charlotte, because of the way our city is spread out and the number of communities and houses available, agent open houses are not a big deal. Most of the companies in town stopped doing agent tours at least six/seven years ago, mainly because so few agents would attend. Personally, I don't attend them. If I need to preview a particular home for a specific client, it's easier on my schedule to call and make an appointment to preview at my convenience instead of adjusting my schedule to the date of the agent open house. The other thing to keep in mind is that the busiest agents (who probably are the source of most of the home buyers) typically don't have time to attend these. Often the ones who attend are those who have fewer clients to look out for.
With so many homes now listed on the internet with multiple photos and virtual tours, agents can get a good feel about a house without taking time to drive to an open house.
But as I said up front, this is a very "local" issue just as most of real estate is very local in terms of conditions and practice. If you are in a community where this is the norm, you should allow your agent to have a broker open house. But if you are in a community where it's not the norm, it probably won't "buy" you much in helping to get your home sold sooner rather than later.
Hope this helps. Nina