If apps are required to provide a reason for using high level of CPU for an extended period of time, then this would help the user decide whether to quit the app.
If you use an app to give you functionality A and you derived it usually costs X resources. Now it is using Y>>X. You ask it why.
It might not be able to answer, since it could be unintended, a bug, something outside it's control (os level, other app contention) changed. So it responds 'I don't know'.
It might be able to answer if the behavior is intentional and part of the systems knowledge space for the current parameters. So you'll get a response like 'I'm optimizing the A experience'.
Either way its not much more actionable than just noticing the App is using more resources.
Now you toss it on a dual-core 1.4Ghz box with 8GB RAM
Should it use 65% of one core? 95% of one core? Something else?
You cannot possibly test every combination out there
The best you can hope to do is verify minimum system requirements (2GB RAM free, 1Ghz dedicated CPU, etc) and hope for the best
I'd really like to see applications have to provide a reason for any case where the UI freezes up, or when an application makes you wait. Sometimes you have no idea if you are waiting on a process that will complete or not complete and have no idea if you have to wait 5 sec or 5 hours.