If there are dozens of units listed for sale and all of them tend to have open houses at the same time to catch cross-over traffic, you might see hundreds or even thousands of visitors at the same time. This obviously will not be pleasant for the buildings’ residents and will be extremely annoying to any residents who aren’t selling their homes. Furthermore, large numbers of walk-in open house visitors can be serious security risk for the building. It’s not uncommon for visitors to start exploring the rest of the building after attending an open house. Since there is no vetting at all for walk-in visitors to a general open house, this can be hazardous for the building.
There have been stories published from time to time about robberies and thefts taking place at higher end, luxury listings’ open houses. There have also been well known articles published about actual combat taking place between brokers and the FSBO sellers they solicited at a FSBO open house.