During that same time New Zealand have picked 4 players as spin bowlers in test matches. Part of that is the presence of Daniel Vettori, probably the best New Zealand's ever spinner to have played for New Zealand. A couple of other players have been named in squads, or played short-form cricket, but Vettori, Patel, Martin and Todd Astle (one match) have been the only spinners used in the last 5 years.
Now in the squad for Bangladesh is the name Ish Sodhi. There's a chance that he's not going to get picked, and that New Zealand will continue to just use Williamson and Martin for the actual tests, but the decision to go with Sodhi is a strange one.
Sodhi looks like he could become a very good bowler. He's got a lot of people who know a thing or two about cricket talking, and genuinely turns the ball. However there's a difference between being able to turn a cricket ball and being a good spin bowler. The two are related, but they are not the same thing.
Sodhi has played 12 first class matches for 22 wickets at an average of 48.40. That's not a very encouraging record.
To be fair to the selectors, there are not many New Zealand spin bowlers with a great record recently, but Sodhi is the player with the worst record of the current crop.
Here are the records of all the potential rivals for the back-up role to Bruce Martin since 2012: (assuming Vettori is unfit)
Jono Boult, Bhupinder Singh and Nathan McCullum have probably not played enough first class cricket in that time to really be able to read much into their records, but there is a name that stands out as a glaring omission: Jeetan Patel.
While I was one of the people who said things like "he should never play for New Zealand again" based on his somewhat insipid performance with the bat in South Africa, the selectors job is not to pick someone to face Steyn and Morkel, rather they should be picking the player who is the best equipped to take over if Bruce Martin gets injured. For me that's Jeetan Patel.
Bruce Edgar said that he picked Sodhi based on his good tour of India with New Zealand A. A tour where he took 2 wickets at 84.5. Astle ended with better figures in each innings that both bowled in. If Sodhi was being taken as a third spinner, then that would be a little more understandable. He's there to get experience, to get exposed to some quality coaching etc. But the risk of Martin getting sick or injured and having to thrust a player who has not yet earned his spot into a test match (against a team that is good against spin) is too great.
Cricket is littered with stories of young players who are picked too early, and either never recover, or take much too long to do so. It is one thing to pick a young player who (like was the case with Vettori) has put sufficient performances on the board to suggest that they deserve their spot. It is another thing to pick a young player who has yet to do that. It sometimes works (Doug Bracewell is a good example) but often doesn't. Martin Crowe has said repeatedly that he should not have been picked so young, as he hadn't yet convinced himself that he was ready to step up. Spin bowling (and legspin bowling in particular) takes a certain psychological strength. You have to be prepared to be hit, relying on your own ability to beat the batsman in the end. If a player doesn't believe in themselves, then the best laid plans are often irrelevant.
I believe this is a bad decision by New Zealand cricket. It is a big gamble with a promising young player's career, and also a gamble with a potential banana-skin test series.