First, I do want to dispense with the notion that Martin will, as his contract winds down, be well-suited to DH'ing.
Because, no.
Even after his outstanding 2014, with the best numbers of his whole career, Martin's OPS+ over the last three seasons is just 108, which is actually a bit higher than his career mark (103). He's not going to improve with age, so we wouldn't expect him to be even average when he's 34 and 35. Sure, he might be a passable DH option against lefties. But even then, just barely.
No, it's Martin's defense that will keep him in the lineup for all four years of this new contract. And more specifically, it's his pitch-framing. We're all still struggling with how much credit for these things should go to the catcher, and how much to the pitchers. But according to Baseball Prospectus, Martin's been one of the most accomplished framers, ranking second in "extra strikes" in the majors over the last seven seasons.
One caveat: That's a counting stat. If you account for opportunities, Martin would rank FAR behind Jose Molina, Pitch-Framing Master. He would also rank behind Jonathan Lucroy, and maybe Ryan Hanigan and one or two others. But based on the data we've got, it's hard to avoid the conclusion that Martin is one of the very best at this now-acknowledged and semi-quantifiable skill, and it seems he's worth somewhere between one and three extra wins per season, based on just this alone.
Which is how you can justify paying him $16 million per season. Without the pitch-framing, you just can't do that.
