Since Spirit shareholders will be paid $33.50 per share, why not load up now at $25?

Other than the possibility of the government not allowing the merger, I don't ever understand why stock prices don't get closer to the buy outs once they are approved by the shareholders. What is the risk of me adding a lot more SAVE to my portfolio? It would seem they at least have a back-up plan if it doesn't go through and Frontier is still interested.