Do you aware that pitching in the last inning is something else than pitching in the 7th or 8th and mostly in close games and/or pressure situations? Do you? If not, do not keep reading, would be worthless the discussion.
My point all along is that Papelbon is not an experiment. He's a stud in that role. Look at his career numbers. IMO He is in that famous Mo's highway . He's been consistent and durable for 9 yrs man, 9! And still young and with a lot experience under his belt ... And not to mention and mostly already proved in your own environment. Notice that I'm not saying that Tazawa couldn't do the job, but to me would be another experiment. He is an unproven closer in the end. I'm not sure about you, but I'm done with experiments. Look what happened with Bard, Aceves, Etc. and in the meantime those experiments and bad decisions had cost us a lot of money-- could have used on Pap's contract--, prospects and most important, wins.
Again that is my opinion.