I think it would take a lot away from the draft. I'm not necessarily opposed to the idea, I just think it would diminish draft night a lot. I think it would also greatly affect free agency. Much more so than a sport like the NFL.
Take Boston, if it didn't think it could draft Williams at 27, might it have taken a flier on Noel in free agency. If Boston had Noel locked in, would it have still taken Williams or would it try to move Noel or Williams.
I just think a lot teams just wouldn't sign a bunch of the mid-tier or lower free agents till after the draft to avoid that type of situation, which puts teams in a much worse position then now (because a lot of those signings wouldn't happen till after summer league started). Then what do you do with summer league, do you delay it or just keep it the same time but with a lot less time to prepare your rookies.
Sure, for the teams chasing James, George, Cousins, etc. it would be nice to know their decisions before the draft, but I think it really is only those top tier guys that would have any real benefit to the draft being move. So for a team like the Cavs, knowing James decision before the draft would have had a pretty dramatic effect on what they could have done in the draft. For example, if they knew James was going to stay they could have more easily moved their draft pick for Kemba Walker, or something like that. Or on the other side, if they knew James was leaving they could have moved Love for a draft pick, like Love for 4 and Parsons, or something like that. But outside of those few players/teams, I don't think moving the draft until after free agency really matters and I do think it would cause a lot of issues for the mid-tier to lower free agents.