Last I checked they still hadn't allowed everyone to simply join, they're still doing this slow rollout. And there's no Mac client either, which is pretty stupid considering its been on limited release for more than two years and its been promised for longer. I'm pretty sure Valve has moved too slowly to best LoL at this point though. It's pretty crazy how popular it is especially for a PC title.DoTA2 is free to everyone now.
That's definitely why a single free to play version of Halo on PC would make sense to me. Port over maps from the entire library over time, offer weapon skins and armor as paid content.