single sign on from any machine in the house, roaming profiles so all of my documents and settings are the same regardless of where I sign in from, exchange requires it, I can attach other services to it (like my web server) so I only have one password for everything on my network, I can control all of my machines automatically with group policy... there's about a thousand reasons to use it, if you're not then you are missing out on 80% of what's good about microsoft