The larger you get the more and more likely you will want to be routing and VLANing your locations. Many people have found out the hard way (ourselves included) just how bad a large broadcast (layer 2) network can be. One user can cause a broadcast storm that can take down your entire network. If you break up your network into smaller segments, you can dramatically mitigate the effects of this.
I guess the bottom line is if you plan on expanding and covering a large area with a lot of users, you’d be much better off getting the infrastructure set up properly while you are small, because the larger it is the more complex the switchover will become and the more downtime you will likely have during the upgrade.
To give you an idea - we’re a pretty small WISP, servicing around 250 customers. We started noticing problems with the flat network design around 150-180 users. One user was infected with a worm and the flood of traffic effected the whole network. Once we were able to shut the user off (turned off the ethernet port), in a matter of minutes the network returned to normal operation. We have about 7 Canopy APs in our network and 3 generic 2.4GHz APs we are slowly switching to Canopy.