·Time Warner Cable
·Verizon Online DSL
The FCC can't add more 2.4 spectrum...and more 5GHz is good
...because it's already licensed out on both sides of the band. On the upper side, it's a company (I want to say Immersat or someone like that). On the lower side, I believe it's other services that can't be easily moved...and routers don't support below channel 1 anyway.
Also, think for a minute about semi-rural areas that are just out of reach of cable, and are being abandoned by DSL providers (or at most being provided low speeds at high prices). These customers can be reached via 5GHz wireless in many cases, and more available spectrum in the band allows for more capacity/less interference from competing networks. So maybe your WISP can, for customers a couple miles or less from the AP, run 40MHz channels and offer 30 Mbps down and 10 Mbps up, consistently, where cable won't go, where they'd be able to only offer half that with less available spectrum (due to running afoul of other 5GHz users).
As for 2.4 GHz congestion, sure, it's there. There are twelve distinct networks visible at my location, and to my knowledge they all use 2.4 (too lazy to check). However I have no issue pushing 50 Mbps over my WiFI (also in 2.4). Then again, I have a router that's built for high RF performance (600 mW output...though I have it turned down to around 200 mW) so I don't get dropouts due to lousy equipment on either side of the link.
I have 68 other WiFi networks on 2.4 Ghz. All I did on 2.4 Ghz was print and stream music on my 4th Gen iPod Touch. Even just streaming music was horrible. It will cause buffering. I had to keep switching from channel 6 then back to channel 3 back and forth because of interference.
I then bought a new iPod Touch and a new dual band router and I now have no problems streaming music on the 5 Ghz. My new 5th Gen iPod Touch is the only thing that I have that works on 5 Ghz. Even my Dell XPS laptop from September 2011 only has a crappy 2.4 Ghz wireless so I keep it plugged into Ethernet like anything else that has a Ethernet port is hardwired.
While sure not a fun RF environment (especially if they aren't nicely laid out 1-5-9-13 or 1-6-11), I bet 2.4GHz is workable. Look at something like Ruckus, their AP's will run standalone (controller isn't needed) and will handle a high noise environment far better. Cost is about $600.