Call me crazy, but I think Melo is Ainge's PLAN C.
Plan A is obviously draft + sign Griffin
Plan B is draft + sign Hayward
But I think if neither are available, he'll go this route, especially with Melo's trade price LOW.
You could still draft, and maybe be able to add someone like O'Quinn to the deal as well, then keep some guys on Bird Rights.
Of course, the alternative plan is to just draft and do NADA, basically just keeping all of your current guys including KO, Amir, etc., BUT the C's coaching brass/front office like Melo too for the most part (so they'll be no doubt interested in him as an option).