Re: O vs D benefit, I think you're missing the forest for the trees. I find the best thing about these games is their inherent flexibility–you're constantly assessing risks and changing coverage (maybe less so in hotbox) to play successful D. This has obvious carryover to team defense–zones benefit a lot from a player who doesn't see just "his" man or the disc, but looks beyond that. Similarly in man D, opportunities to poach or help on D are plentiful, but you need players who are prepared to be flexible and take some risks to pull it off consistently and effectively. You can take lots of risks in free play, baiting, experimenting, and improve the risk:reward ratio for in-game.

I think the "rules" of defense are exactly NOT what free play reinforces, and that's a good thing. We can teach things like positioning fine in practice. Free play helps you to develop a sense, say, of at what point a thrower is DEFINITELY going to throw to where they're looking, or what kind of spatial margins are defensible and what ones are not. I tend to think a lot in terms of space, on O and D, and I think boot (note: we play the "SF variation" at Dartmouth) in particular did a lot to develop that sense. I'd argue that those habits you develop in hotbox or otherwise don't supplant but rather supplement the ones you learn on an ultimate field.