You're likely on a "business" plan anyway with a T1.

I'm on a similar plan.  I pay for my bandwidth and have guaranteed thruput.  Unlike most home plans, my only limit is my relatively slow ADSL speed (1.5MB down, 868KB up).  While I rather doubt I could exceed 64 DVD's worth of downloaded material per month, especially since I get my entertainment media out of a computer hooked up to a DirectTV satelite dish, there's nothing in the fine print of my contract preventing it, only the throttle on the ADSL line, and I can have that increased if I pay for it.

250Gb is enough for a home user ... You are using your home connection as a work connection. You even admit that.

Perhaps coding in ruby has diminished your critical thinking capabilities? (FFS ... you even quoted the very first line!)

If the foo shits, wear it!

250GB is more than enough for a home user

Often our clients will just buy the paper licenses and let my company...

Minutes and seconds are easy to understand. Phone calls are easy to control. Devices switched on and off are easy to control.

Megabytes aren't. WTF is a megabyte? One minute of video? 100 web pages? If someone sends several gigabytes of data to my PC it hasn't requested, how can the ISP detect that?

Most medium to high end routers and switches have traffic counters so that you can do diagnostics.  It shouldn't be a big deal for the ISP to interrogate the switch you get your service from for how much traffic you've been making. 

And if you care, you can always configure your low end firewall to keep track of how much traffic is addressed to it.  Note that I did say that you should be billed by the Gigabyte.  Perhaps it ought to be based upon how much outbound activity comes from your node, since you can't always control what comes in.  High school bullies could organize a large group of kids to send substantial amounts of traffic to some particular household, and they'd have to pay for it, even if it wasn't asked for.  So we're looking at upload traffic and acknowledgements. 

This would also be an incentive for people to secure their home PC against malware.  After all, malware could easily raise your bill. 

You, as an end-user can control how much water, electricity, gas, and telephone service you use.  You can even control what time of day it is used.  So I humbly suggest that this sort of billing might not be such a bad idea after all. 

Why should heavy users pay more than light users? Is it necessary for a profitable model?

Heavy users use more resource.  It depends on the specifics and costs of provisioning.  For a phone, most of the infrastructure needs to be put into place, just for availability.  So for local calls there isn't enough time-based cost adder to put in the expense for billing based on it.  Long distance includes a pinch-point and the economics are different, so they do have usage-based billing.  Incidentally, we have tiered billing, rather like what they're talking about for cable.

What about driving on the roads. Should your yearly registration be mileage based?

Don't forget that excise tax on tires and gasoline tax are both roughly mileage based.  People who drive more do pay more in taxes to pay for the roads.

Should your plane tickets be based on your weight?

Maybe.  They're already getting more stringent about checked baggage, and it's all about the weight.  As the fuel gets more expensive, they get more uppity about weight.  They've probably avoided weight-based ticket prices because that's a legal minefield of discrimination lawsuits.  But yes, it costs more to transport a planload of heavy people than a planeload of light people, given that the basic unit is the pair of buttocks, not the pound.