If you look at Williamson's form since Christmas, it is pretty awesome.
In the 50-over games since December 23, he has scored 107*, 70, 108*, 77, 81, 1, 84 and 69. That's 597 runs at an average of 99.5. He also has 11 wickets at 22.5.
In the f/c games since the Christmas break he has scored 529 runs at 105.8.
I know that stats can be manipulated however you like, but these are still pretty useful, however you look at them. I also realise that NZ domestic cricket is a big step down from Test cricket, but what else can you ask the guy to do?
One problem NZ cricket has had in the past is that we have either picked guys with loads of runs but dodgy techniques (e.g., Sinclair, Ingram, Fulton) or supposedly good techniques but limited first-class runs (e.g. the Marshalls, Watling).
The potentially great thing about Williamson is that he combines prolific run-scoring with an allegedly solid technique.
One other point: McCullum was kept out of the Test team for a while by Robbie Hart because the selectors didn't want to drop him in the deep end. Ross Taylor was held back for the same reason for a while. In retrospect, both of those decisions seem silly.
As Bradburn said, would you rather Williamson get his taste of international cricket on a docile pitch that he knows well against a good, but not fearsome, attack, or on some hot, dusty Sri Lankan wicket against Murali, Mendis & Co.?