It does not make sense to force the battery to discharge before the range extender starts up. The most efficient use of the engine is during highway miles, so why should I have to drive highway miles on battery? If I need to do a trip of 100 miles to another city, then drive around that city for 30 miles, I would rather have the freedom to select gas operation once I hit the highway, where my MPG will be highest, rather than ending up driving for 30 miles around town on gas getting less MPG. Overall I would use less fuel, right? But we are not smart enough here in the states, I guess.

"If anything Raygun was a Union busting, oil-lover."
There, I fixed it for you.

The 50MPG vs. 40MPG has a break even point of around 100 miles, less than that the Volt wins with it's 35-45 mile electric range, above 100 miles, the PIP starts to come out ahead.

GM has mentioned that they are talking with utilities to re-purpose the used batteries as grid buffers/storage once they are removed from cars.

70% is about where the available range would start to be reduced, but I assume one could continue to drive the car with a reduced range, and avoid replacing the battery until the range was less than you were comfortable with.

Oh, and the GM replacement battery cost is currently $2994.64 (price is in their parts system @ gmpartsdepartment.com). This for out-of-warranty replacement, without labor or core charge of course.

If you mean mechanically drives the wheels, this only occurs at some high speed, the number varies, but commonly I have heard 65MPH bandied about for when this occurs.

You could switch to Mountain Mode to get some charge back into the battery, or to save some battery charge for later.
The Hold button on the Ampera is even better. It allows you to say, warm up the car using the ICE to avoid draining the battery just for heat, or to use the ICE on the highway where it is most efficient, saving the battery for in-town driving. Apparently we are not smart enough to use such a function "properly" here in the U.S.