I really think the main point of the new tiered plans is to make people care about the data they use. As it is right now, I sit in front of a computer all day as a software engineer. I stream pandora to my phone because that is the easiest and when i get up to talk to people I just leave it streaming. When I get home I use Wifi but when I am at a friends house I don't because I don't feel like setting it up and connecting (when they have passwords etc) and 3G is just fine for what I am doing when I am out. But if I were going to pay for extra data in a tiered plan, I would make sure to connect to Wifi whenever available and I would stop my pandora from streaming when I step away from my desk (Or even stream from my work computer).
I Really think this is what they are trying to do. Right now we use data whenever/wherever we want..for whatever we want and don't try to conserve it at all. This is why networks are so congested and once people start to care about the data they use, that congestion will go down.
I'm not a fan of tiered plans either (Since I use a ton of data) and I do think it is a way for the cell companies to get more out of their network without having to invest money in it...but it was only a matter of time and I don't suspect it will stay around once data gets faster and cheaper.