I have an Acer laptop - getting on. Battery totally dead so it's now a de facto desk top pc.
From time to time you HAVE to reboot, depending on your installed software. Norton often requires a reboot to settle updates for example.
If your RAM is a bit low, or you have too many programs in your start up menu then rebooting can be a time consuming pain. It used to take me ages (I run XP, not Vista) so I avoided rebooting if I could help it. Then I got 1GB of Ram on top of the original 256GB and boot up time halved.
I was advised to run a weekly disk check on my pc to keep the hard drive in good nick. This requires a reboot. I don't know if you have to do that on Vista, but I was told it was sensible practice for my laptop.
I'm not very techie, but I do know that a reboot can free up resources that haven't been released by some program or another that you've been running. I think the proof of this is that I cannot switch users back and forth on my lap top unless it's recently been rebooted. It runs out of resources after a while and freezes on a black screen when I try to switch back and forth.
I set my laptop to sleep when I close the lid. Opening the lid starts it up again - and I just need to enter my password. I tend to save my powering right down for bed time when I won't need to use the laptop for several hours. It can be booting up again while I'm getting my breakfast ready. As I have dial up there is no point in leaving the pc on all hours to do automatic updates since no one can phone me while I'm using the net.