Don't you get a Live account when you sign up for Hotmail? Those don't get deleted after 270 days; I've used my Live account (originally a Hotmail account) for Xbox Live and other services without logging into Hotmail for years now.
According to the docs and the developer comments they do get deleted. Presumably using the account with xbox live and other services is what's keeping it active, even if you don't log into hotmail directly.
I've also already experienced both situations. When the @live.com accounts became available I registered my name, never used it, eventually forgot to keep logging in to keep it active and it got deleted. Meanwhile, the account I use for xbox live stayed active even though I hadn't logged in to hotmail in at least 2 years until a couple weeks ago.
So I doubt it's for usernames.