Well, if you check out maps, there are definitely areas where you cannot get a signal. Mostly states in the midwest.
The purpose is that where it is possible to get a signal, the signal is not messy. Since, cell phone reception typically suffers in crowded areas such as cities and other high residential areas despite that GSM is supported in the area.
You see the ad on TV for Verizon with the guy saying "Can you hear me now? Good." Leads me to believe that you can be heard anywhere. I don't think this is entirely true. On the Verizon site they ask for your zip code or phone number to see what services they can offer. I get a message back saying "Verizon does not offer local phone service in the area code you provided."
I have also seen other brands of cell phones useless in some areas if you are not in range of one of their towers. But yet they each claim to be the best.
I understand that cell phones are regulated by the FCC at a very low wattage. So if the signal cannot get out, how can they get by with their type of advertizing?