The only thing that kills battery life in Android is something using the cpu, cell radio, bluetooth, GPS, etc etc continuously. An app that people think is running in the background is not actually running. It's using uncached space in the system memory because android see a usage pattern with that app. If it didn't do this it would be a slower start-up then everyone would be complaining that their phones are slow.
The bottom line is you can't say that an app improves battery life on your phone when you have a malformed app that isn't running correctly with the system. That's what most of the "experts" call a bandaid fix. Next time your alarm doesn't work just download a different alarm app to fix it.....does that make sense to you? If you have an app that doesn't perform correctly with the android system it's not the systems fault and the only way to make sure that apps do work correctly is for the system to take control of the services running and do it's thing. IE Froyo.
In the end Auto Kill is the only thing that doesn't work, and ATK hasn't be written, at least the free one hasn't, to work with Froyo correctly. System panel still shuts services down, they don't start back up etc.......but yeah the "feature" has been removed..........
We can argue ad nauseum whether an app is actually running or not. I don't know the internals of the Android OS so I couldn't tell you one way or another. But I CAN tell you without question that my battery drains considerably faster with 2.2 and takes significantly longer to charge with 2.2, with all other apps and settings being equal. I have EXACTLY the same apps running on 2.2 that I had on 2.1, and after factory resetting my phone (twice) I have EXACTLY the same settings that I had on 2.1. So the ONLY thing that has changed is 2.2 vs 2.1. Even under 2.1, although I had ATK installed, I never used autokill and only used ATK to kill some memory hogs like maps, navigation, google earth etc. I have run 2.2 with and without ATK and battery life is worse than 2.1 either way.
So I guess my point is I wouldn't mind them removing the API that allows you to kill tasks, if they had fixed the underlying problem that was necessitating it's use in the first place. I agree with you that a task killer in the wrong hands can do more harm than good. But task killers didn't spring up and become popular for no reason. They sprung up and became popular because there was a need for them. Because there was an issue with battery life and ATK made it better. Now to use your analogy, with the "upgrade" to 2.2 they took away the "band-aid", but they didn't do anything to fix the wound that it was covering up, in fact the wound has gotten worse and now you don't even have a bandaid to cover it up.