I've written several times about how the long boot time on a normal operating system needed to be trimmed (I was looking at Windows, but it could have applied to any), especially with the rise of instant-on devices like tablets and smartphones. I even installed a solid-state drive to speed up the process on one of my machines.
I've since built a lower-power computer, one that draws almost nothing when sleeping and now I only power down my PC when I'm away for a few days, otherwise it's either on or asleep, never more than a mouse wiggle or random key-press away.
The reason for the change is simple: speed. Even with the developments in boot times, turning on a computer is still a long process. Several minutes at least. I could have checked my email, news feeds and a ton of other stuff in that time on a tablet.
Using sleep mode means my machine is completely usable in a few seconds (almost as fast as a tablet but not quite) and I can put it to sleep in about the same time. It's an area Windows 8 has brought improvements to (as it has to boot times). By the time I've entered my password on the lock screen the system has already checked my email and begun grabbing updates where it can so I don't have to wait.
The added benefit of not starting my machine means it always opens in the desktop too, so I rarely see the much maligned Windows 8 home screen that has thrown so many who have moved to the new OS.
If Microsoft could work on reducing the need to reboot after every update (no matter how small, it seems) I'll be laughing.