In short, he should never have been allowed to play as a wicketkeeper, as he did for just over half of his 90-odd tests so far. His test average is creeping towards the 40 mark, but it stagnated in the mid-30s for too long, a return that is light years away from what the man is capable of, as we are finding out now.
Having McCullum keep wicket for a large part of his career was like getting a superior racehorse to don a plough. Batsmen who can keep wicket do not bat as well when they are also parked behind the wickets. McCullum's batting prowess was too precious to be treated that way, not to mention the crime of hiding such a talent down the order of a NZ side.
There is no point in comparing McCullum to someone like Adam Gilchrist, who played for a great Australian team loaded with world-class batsmen. Gilchrist was able to do the double act, his batting averaging in the high 40s.
In McCullum's case, the evidence is now overwhelming. A few years down the track of his post-wicketkeeping career, McCullum's batting has gone to a completely different level, even though his back is in meltdown.
That fragile back got one heck of a workout in Christchurch, as the little blaster smashed the Sri Lankan attack with a savagery that produced a ridiculously high run rate almost all the way to a double century.
The tonic for McCullum's batting, according to the coaches, is a better understanding of what he needs to do defensively. If only the solutions had been found a decade ago.
McCullum should never have been made a jack of two trades. He always had extreme potential as a batsman, and is now probably exceeding what his biggest fans thought possible.
What McCullum did to Sri Lanka was off-the-planet amazing and absolutely thrilling to watch. This humble man, and obviously inspiring captain, has given us a year of unbelievable batting. And yet as he laid the tourists to waste, it was time to reflect on his career and think not only of what might have been, but also of what has been wasted.