The signing is only baffling now because Ainge was forced to alter his strategy mid-stream.
Hate to re-hash this: but if Cleveland doesn't win the lottery, they probably aren't able to trade for Love, and we have the best offer - all of a sudden we have Rondo-Bradley-Green-Love to build around, which makes sense. Bradley is ideally probably a 5th/6th player on a contending team. With the salary cap about to rise in 2016, $8M isn't all that crazy.
Of course, now that we've been forced to blow everything up, he probably isn't the best fit. But he's still developing, and we need to pay somebody eventually or else we won't hit the minimum salary pretty soon. We could do much worse.