You can't save more power than a device that's off...
That said, how about a linux-based phone that is completely powered off and wakes up when it's called? Would you want to have the caller hold for 45 seconds while your OS boots to start a voicemail application?
Think about servers with a 99.999% uptime guarantee. with a 5 second shutdown and startup they can be rebooted 20x a year. with a 45 second shutdown and startup you can reboot them twice a year.
You obviously haven't used a computer that boots in 5 seconds yet. I guarantee that when you do, you will never want the old 45 second boot again :)