I wonder how many of the 25 million are people squatting on their family names and such. That's what I've done. I login once a week or so to make sure the account stays active.
I think it's 90 days for hotmail/outlook, which is pretty short compared to gmail, which I think is a year. But, that would also explain why it's a lot easier to get the email name you want on hotmail/outlook.
If you're using a multi-account IM client(Adium/Pidign/etc') you can add the account there, and it will stay alive as long as you continue to use the client.