As for the Mixtapes themselves, I'm still kind of reluctant to jump all-in on the Copyright free-for-all that Rap likes to work with (e.g. Tarantino's Kill Bill siren) and occasionally gets called out on. I think retaining some kind of performance rights and compensation standards - think 2Pac's version of "Changes" - is just fine for the useful progress of the arts (as Copyright intends).
Besides, I'd be pretty pissed off if a shitty rapper used a track of mine and started blowing up without cutting me a piece of the action - that biting shit won't fly with me, even if it makes me sound like Don Henley. Now, if approached by somebody I respect and admire, say, Chance, I'd have to think about signing for free or not. One situation apparently went down kind of like that with Chance and Apollo Brown, in that using a beat for free was not approved by the respective camps[1]. Hip-hop showing respect and doing good business is nice to see.
[1] http://uproxx.com/smokingsection/chance-the-rapper-apollo-br...