I presently limit my wireless guests per-connection. But they could still gather up a lot of connections and gang up on my bandwidth. So yes, I could see how this would be useful. Limit the client connections using the usual means, but also put a cap on the whole dang' thing to prevent an overflow.
It would be great to rate limit a topology or WLAN (SSID). As a convention centre we could provide customers with a level of bandwidth to be divided up between their devices. My topologies are all bridge@EWC but I have the ports trunked so rate limiting on the switch port would affect all WLANs