Airtime fairness helps a lot but if a 11b device connects, you automatically lose 1/3 of the throughput even when that device is not active. 11g can also slow down others but only a couple percent. Also, if you disable 11b rates you can have more SSID beacons (more networks) in parallel as the beacon is then transmitted at 6Mbps PSK instead of 1Mbps DSSS.
