Really surprised at this personally. Franklin actually has an excellent record when it comes to playing overseas in a variety of conditions. In both forms of the game his away record is markedly superior (test avg of 24.7 away compared to career avg of 28; ODI avg of 30 away compared to career avg of 37). He's had good success in England, South Africa and the Sub Continent (admittedly only against Bangladesh). Admittedly he's more effective on green wickets, but name me one fast bowler who isn't? More importantly he's the kind of bowler who can get a lot of swing no matter what the conditions (provided its a red ball he's swinging that is). He's also got an excellent control of reverse and seems to be gradually building up his pace. But overall what makes him valuable is that he's a wicket taker. His strike rate, though it may surprise you to here it, is one of the best of any international bowlers currently playing in the world. The fact that his batting average is in the low 20's and climbing is a nice bonus as well.
I agree with other comments that Martin is the 4th bowler in the New Zealand attack, but I don't necessarily agree that Mills is an automatic selection ahead of him. Mills is by trade an opener, who greatly relies on the prodigious swing of the new ball to pick up most of his wickets. Martin, while not necessarily as good in this role, is faster and more consistent, arguably making him more suited to the role of first change, which requires longer spells with less swing available. When the two have played together in test matches, neither one has dramatically outshined the other. It's a shame that Mills won't be fit in this series, as that would give a good idea of who deserves the 4th bowler spot. I certainly don't see how anyone can say that Gillespie is a much better bowler than Martin seeing as they play alsmost exclusively in different forms of the game.