New
#11
1). Cost (licensing 2 copies of the OS)
2). Space consumed on the hard drive
3). Unnecessary complexity
4). Not likely that any performance increases would be noticeable by the person
5). Maintaining 2 OS's (defrag, viruses, spyware, malware, software installs).
In my particular case of having 2 machines available...it's nice because I can continue to use my primary box for whatever I want, while the kids simultaneously use the older computer to play their games and surf the web, etc.