I know cetain people on this forum love Nathan McCullum, but unless he starts taking wickets, his bowling is of little help to New Zealand. Yes, he concedes a modest run rate, but his bowling average is over 50, and he's taken just 12 wickets in 19 games. Opposition sides are perfectly happy to tick the run rate over at 4 or 5 in the middle overs, provided they don't lose any wickets. Taking wickets breaks up momentum and reduces the amount of damage a side can do in the death overs. If McCullum continues to concede between 45 and 55 runs per innings, and manages a wicket only every couple of matches, then he's really only doing what any part timer like Ryder or Styris could do in a similar situation.
Maybe his wicket taking ability will improve when he gets to bowl in conjunction with Vettori (which he hasn't really had the opportunity to do in recent months), but it'll have to if he wants to stay in the side beyond this world cup. You can say what you like about Jeets, but he picked up more than a wicket a match on average, and this is what McCullum needs to do (at the least) if he is to be of value to the Black Caps and make a meaningful contribution at this tournament.