I think the trouble, Mr. Cullen, is that you're conflating two things. You're conflating the composition of Parliament and then the composition of government and the policy that results.
It is true that if we have a PR system, we'll have a composition of Parliament that more accurately reflects the party preferences of voters. That's a normatively good thing. I take no issue with it at all. The principle of good is another issue.
But then governments are formed. The point is that it can be a single-party government, as is now the norm in New Zealand, or it could be a coalition.
By the way, since New Zealand changed to MMP, it's now a single party that typically rules. They don't even have supply motions supporting them anymore, so that's worth noting.
The point is that the policy output is something else entirely. That's a debate in the academic literature, on which I think the evidence is actually relatively muddled, about whether the policy that comes out of government is closer to the preferences of voters under PR or first past the post. The principal reason in majoritarian countries is that single parties can move to the policy median swiftly, but they're not able to when they're bound by coalition agreements.