Nice chart Rob. The main reason for getting XP Pro over the other 2 would be the ability to join the computer to a domain for central admin reasons, namely group policies. XP Home is good for most users that do nothing but turn a computer on and use it and turn it off again. MCE is a great solution for media nuts and gives you the extra file security that Pro has and Home lacks.
I was doing a bit of contract work for a company and was working on setting up a domain and an exchange server for them. They bought all these nice new computers for their employees and I was going to join them to the domain so they would have central logons and exchange email accounts and all that cool stuff and they all had XP Home edition....
Looks to me like the only reason to pick XP Pro over MCE is if you absolutely have to join it to an Active Directory Domain.