You're absolutely right, but defining "upgrade" depends on the person. A lot of people these days don't want something bigger than they've reserved. Personally I don't really care, I like to try out something new whenever possible. Anytime I rent I'm usually just driving to and from the airport, so even with errands I'm driving 60-70 miles tops. Even at our ridiculous Canadian gas prices, we're talking a difference of $7 in fuel between a Yaris and a Durango (to use extremes).

That said, some of the nicer upgrades I've had have been in-class. I had a compact booked with National at YEG and they gave me a Focus. Rather than just any old one, they must've recognized I was Emerald Elite since they gave me an SES with Sirius, Sync, heated leather seats and moonroof. I liked being able to have all the creature comforts I love and still have something easy to park and cheap on fuel.

So it depends on the person. I'll sometimes be inclined to take a minivan if offered as a "pay it forward" to my fellow travellers. Whatever they give me, it still beats either waiting for 30 minutes in -30 weather for a cab or being crammed into a 15-passenger shuttle van that decides to skip my drop-off point to save time.