It was completely understandable, to an extent, that the Yankees did not want to jump at re-signing David Robertson. The organization never particularly trusted him in the role--they even signed Rafael Soriano at a premium to fill the closer role in Mariano Rivera's stead--and they just weren't sure if he would thrive. At this point in the season, it's pretty clear that Robertson is unquestionably a Proven Closer, whatever that is. So, should the Yankees re-sign him after his walk year? Robertson made it clear in a recent interview that he would no longer re-sign for the hometown discount. You can pretty much consider him like any other free agent at this point.
I don't like relievers, and I'm not very shy about it. Proven Closers are no sure thing, and we see dominant closers come into and out of the game quicker than it takes for my phone battery to die. Remember when Keith Foulke was the Red Sox shutdown closer? Me neither. Remember when Eric Gagne posted an over 4.0 fWAR season and then vanished into the abyss? Nope. I can count the amount of excellent relievers of the past 15-20 years on one hand (and one finger) --Mariano Rivera, Trevor Hoffman, Joe Nathan, Billy Wagner, and Jonathan Papelbon. I'm not going to argue that Robertson is going to be one of these, but I can reasonably say that he can give them a run for their money.
In terms of durability, Robertson is practically the definition. Since his debut in 2008, he has only missed 105 days due to injury. That seems like a big number, but it's really not. That's only an average missed time of 15 games per year, or about a handful of relief appearances. The biggest predictor of future injuries is past injuries, and so far Robertson has not had one serious one.
In terms of track record, he is among the elite in the past fifteen years. There have only been 14 relievers that have had four or more 1.5 fWAR seasons from 2000-2013, and now Robertson is among them. He has a career K/9 of 12.02, FIP- of 63, and ERA- of 65. If you're going to spend strictly on performance, Robertson has clearly been one of the best relievers in baseball.
There's also an emotional appeal, one that I usually avoid. Robertson is a "homegrown Yankee", one that came up with the system, grew before our eyes into what we know him as now, and he won a Championship with the team. You could even consider him part of a core that made up the younger players of 2009-2012, and that's something I'm sure the Yankees would love to keep at a fair price. There's also the feeling that he is Mo's successor. We all knew what it felt like having him come out of the bullpen--almost a calming presence--and we can see that in Robertson.
I'm not just saying that I want the Yankees to re-sign Robertson because he is a True Yankee or because of some strange aura around homegrown players; the Yankees reasonably passed on Robinson Cano, and we all moved on. I'm willing to break my general rule on falling in love with relievers because I truly believe that Robertson is one of the better relievers of the past few years, and his track record does not suggest he will be anything other than that in the recent future. There's always the risk of bust--like with many relievers--but if you're going to take a risk, it might as well be on one of the best. Get it done, Brian Cashman.