I think the fundamental problem is capacity. I haven't run a WISP, but my understanding is that a highly directional rooftop antenna solves most signal strength problems for individual subscribers. However, there's a practical requirement for the tower to have antennas with broader coverage (I think WISPs conventionally use sector antennas), because otherwise you'd just have too many antennas that need to continually be adjusted as subscribers come and go. Half-duplex medium and protocol overhead means that you're not going to get very many stations on a given combination of antenna and channel before performance starts taking a major hit. I've seen general guidelines that recommend no more than 20 stations per AP; I assume a WISP tower setup can optimize for more stations per channel, but I'd expect that to be on the order of a 2-5x multiplier, not 100x.