They already did for some time, I am not sure if they've done in the scope of this rebrand, it is surely an welcome addition. [1]

Don't expect that it just works with any XMPP client as it uses a proprietary authentication mechanism "very similar to Facebook" OAUTH2.[1] More popular clients have already integrated it.

That's why Windows devs drink.

[1] http://blog.process-one.net/details_on_msns_xmpp_server/