Good idea, implemented badly
I think its technically a good idea, but as mentioned the lack of notification or choice to the user is the issue I think most people have. The technical reasoning behind it I think most people get.
But if you think cars: you get a oil warning light, or a battery light come on.
If the CPU is being throttled then the iPhone knows its being throttled (i've seen other stories around the net with app's showing usage on phone then vs now values). So why not just have a phone performance/health indicator or graph over time. (and maybe a turn on/off switch) this simple 'phone requires a health check' indicator is all that's needed surely.