Now we all know Windows can run natively on Intel Macs. Does Parallles need a virtual machine to run Windows in Intel Macs? I guess not. In a Intel Mac with Mac OSX and Bootcamp Windows, in order to easily access Windows in Mac OSX (or access Mac OSX in Windows), all we need is an OS switcher. So what does the switcher do then? If you are currently in Mac OSX, a tool in Mac OSX can be used to suspend Mac OSX and lauch Windows in Bootcamp (if it's already lauched, wake it up). In Windows, there is a similar tool, which can be used to suspend Windows and lauch Mac OSX ( or wake it up). When one OS is suspended, the tool will hand over the machine to the other OS and the other OS can make full use of the hardware, so that there should be no performance loss. The main difficulty seems to be how to suspend an OS without restart the machine (but letting another OS take over the whole machine) and quickly wake it up.
This is one approach, but you're assuming that people are doing only one thing at a time with their computer. This is not the case for everyone. Most people who use Parallels instead of BootCamp do so because they need to use both Mac OS X and one or more other OSes at the same time.
I can safely say that the reason I use parallels instead of BootCamp is because I only need one program from WinXP and otherwise OS X does everything I need and more. I only use Parallels for a place to develop using VS 2005, and I don't want to have to lose all of my OS X access to do it, so bootcamp is annoyingly inconvenient for me.
