The downside for me - and this is of course just my opinion and not fact, is I don't see him being all that thrilled playing on a rebuilding team. I'm sure he'll be fine to start out having roughly 6 million reasons to be fine, but by the middle of the season when he realizes he's on a **** team playing essentially meaningless games, I can see him being not so thrilled and wanting out.
Maybe we can trade him or maybe he "retires" again and we're stuck with his cap hit for the length of his contract. Probably won't happen and maybe I'm making a mountain out of a molehill because I don't like over 35 contracts on sheer principle, but for me, that's the downside.