Overage fees are nothing but pure evil. They did use to offer capped DSL and my cell phone data usage is still capped, I ran into it this summer as I was watching videos at the cabin but it doesn't have overage. What happens is at 80% I got a text that I'm getting close on my cap. At 100% I got a new text saying my quota is now up, I'll now either get very, very slow internet connection the rest of the month like enough to check email and barely browse the web, or I can pay up for additional quotas. Back when they offered capped DSL it was the same there.
The biggest benefit to a flat rate connection is that it's flat rate. And particularly today when you got phones and tablets and laptops and consoles and smart TVs and whatnot that all like to go online keeping track of your aggregate data usage is not easy. Overage fees are like the credit card model offering you 30 days free credit. How to do they make money off giving people free money? Because people slip up, get unplanned or unwanted expenses and then they nail the suckers. It's just begging to exploit the people who think they can save a few bucks a month.