Call them cheap, but both Gay and Z-Bo's contracts were/are albatrosses that limited flexibility in the new era of the stricter salary cap. They did a great job to get out from under Gay's contract, and I think they'd be wise to do the same with Randolph's if they can get a decent return.
That said, I'm not completely against your proposed trade, but it wouldn't be the first one I'd pursue. It would give us two more roster spots to fill with a million less dollars to work with. On the plus side, Z-Bo is a real upgrade on Bass, and in 2014 his salary drops while the three you have the Celtics trading away continue to rise, so there are some long-term cost savings. If clearing out the guard logjam could make Redick willing to sign here, I'd be more for it. I also think it would depend on how the draft goes.