Omnidirectional antennas (a misnomer, actually; they really should be called "antennas with 360 degree horizontal beamwidth) are not a menace. In fact, they are necessary to follow one of the precepts Brad advocates above: to use as little spectrum as possible and not hog it.

Many wireless ISPs use multiple sector antennas on multiple frequencies at their access points, unnecessarily congesting the spectrum. It is easy to demonstrate that the most efficient use of spectrum and the least interference in a point-to-multipoint system are achieved when each provider uses an omnidirectional antenna and a single channel, rather than multiple sectors and multiple channels.

Unfortunately, many of today's wireless ISPs use the multiple channel/multiple sector approach, with antenna gains and power levels that violate the FCC's Part 15 rules. Why? In part, because equipment vendors -- who get to sell them more equipment if they do this -- tell them to. Another reason is that no one is enforcing any requirement not to hog spectrum, so they just use it all. This approach backfires, however, when multiple providers must coexist -- especially on bands such as the 2.4 GHz ISM band. The 4 Color Theorem says that even if each WISP used a different channel on a directional channel pointed at each given area and the signal did not splatter into others, there STILL wouldn't be enough channels for interference to be avoided unless the number of available channels 4 times the number of WISPs serving the area. However, there's no "4 Color Theorem" for spectrum. So, if there are as many channels as there are WISPs (which can be achieved by using slightly narrower channels than standard Wi-Fi) you can have no interference between them.