I suppose it depends on how well Boston does this year and what the roster looks like after the draft. If Boston is a mid-40 win team and doesn't get all of the possible draft picks, I think adding Kobe makes sense. For all his flaws, he still can be a dynamic scorer, something Boston needs. However, if Boston ends up with all 4 draft picks, doesn't pick up the option on Amir, lets Lee leave, etc. and looks like it is really going to have a down year while the young guys get experience, then Kobe makes no sense at all even as a F-you to the Lakers.
That said, Kobe will either re-sign in LA or retire. He wants to go down as only a Laker for his legacy. MJ didn't do that, Lebron didn't do that, and Shaq didn't do that. The three guys his career is mostly tied to, didn't spend their entire career in one spot. Kobe wants that.