
I find no benefit in dual startup. When I was learning computers, I dual booted windows and linux so I could learn both. Now that I know windows is easier to use for me and my wife, I just use windows. Dual booting never slowed my pc down, but it did take up another couple gigs of space on my drive. As a practical use, I don't like it. But as a learning tool, its almost essential in my mind. New power users need to learn both since unix is still popular in the workplace. However, I find zero use in dual booting windows machines.
My computer is, in effect, a dual operating system machine. However, it is not dual boot. OS X is built on top of an excellent implementation of Unix, with all of the Unix tools and commands available. At the same time, it has all the programs designed specifically with the Macintosh interface, and will even run older OS 9 programs.
I love OS X!
I actually find good use for a dual booting computer myself. My desk is not honestly big enough to support a second PC. My main daily applications I use have more stability as well as more features in some cases as long as they are run on a windows 2000 system. This is my preference of Windows systems for my daily use anyhow. I do however still work with a few very old applications which require a few special needs. One piece actually requires simply some form of DOS on a FAT 16 partition. This is relatively easy so I simply use about 450 megs of one hard drive to setup a boot to DOS(not an emulation). Another program that I have to do periodic checks on for a long time client of mine requires a Novell 3.12 or earlier OS in order to run properly. This makes my life quite difficult as Novell does not integrate with many other things well. For this I actually have a 1.2 GB hard drive dedicated to boot Novell. I tried at one point to have a KVM switch and multiple PC towers at my desk and found I really did not like all the clutter and confusion that it caused.
At work, we utilize a lot of dual boot pc's, simply because it is cheaper to put in a bigger hard drive than buy two separate pc's with keyboard, mice, etc. We use them for testing application on different OS'. The main OS' we test on are Win2000, WinXP Pro and Win98. I personally would not dual boot my personal pc, instead I would purchase an individual box to run, eg Linux. I just don't want the hassle of having 2 OS' on one box.