Take game development for instance, if your code runs a bit faster that's more frames you're rendering per second. This can mean the difference between a laggy, unplayable mess and a good gaming experience on an average machine (a laptop, or a non-gaming rig).
Do we know that most new software runs in batteries? Do you have a reference for that? It would be useful.
Other question is - does a sleeping CPU always win, or are other optimisations such as memory usage, screen usage, and communications, more important? I know for sure that the CPU is not the main consumer of power on a bunch of Android phones that were used in an academic study.
Probably the CPU is not the main power sink on mobile devices (screen and radio are I think). But cranking up the optimization level is free for the developer.
It's not so simple. If you're trading off simplicity in the compiler for that speed,for example, it may be a net loss.
If speed was the only concern, we'd all go out and buy super computers.
You can optimise as much as you like but then John Q Madbugger comes along with their insane Javascript (not looking at Flickr here, no sir) and suddenly your performance is in the toilet.
Thinking over it some more, browsers themselves were probably not a good piece of software to call out on performance considering how much optimization is actually done to increase their performance. The horrid things people do on top of them deserve all the blame they get though, and then some.