Over on Public Address, Graeme Edgeler has a piece arguing for the elimination of dual candidacy. Not for the usual reason of "zombie MPs" - he's perfectly comfortable with them - but in pursuit of "strong local representation":
And I think [Vote for Change's Jordan Williams is] right. The tendency may not be great, but it is a factor. We've never had (under MMP or FPP) the Westminster tradition of crossing the floor (I don't think government backbenchers in New Zealand have ever taken out newspaper advertisements opposing government policy, for example), so the effect might not be as great, but it could manifest itself in other ways. But maybe under first past the post – out of fear of losing their jobs, with no plan B – local MPs in marginal or somewhat marginal electorates were more likely to more forcefully put their constituents' views in caucus, and were able to forestall unpopular changes, or obtain concessions. It certainly seems likely that an MP who, if they lost their electorate, would be out of a job, would take that part of the representative function more seriously.Note that this isn't "strong local representation" in the sense of MP's doing constituency work for their constituents - because they all do that, regardless of their method of election. It's "strong local representation" in the sense of more independent MPs and weaker parties.
The problem is that we've never had this under FPP, even in the sense of advocacy-inside-caucus Graeme describes. The 80's and 90's, where unpopular changes were rammed through regardless, prove that. Part of the reason is that parties have strong internal rules against dissent. Part of it is that New Zealand voters vote for parties, not individual MPs (how many de-selected former MPs have been successful running against their own party outside of a mid-term by-election? Bugger-all). But part of it is also because they don't just choose list candidates, but electorate ones as well. The result is that an electorate candidate is just as much a slave of the party as a list candidate. Display loyalty, and you get to contest a winnable seat. Don't rock the boat, and you get re-selected. Cause trouble, and you end up like Brian Connell or Brian Neeson (and those examples are from National, which is more democratic than most). So eliminating dual candidacy doesn't seem likely to achieve the desired effect, while having significant costs (and not just to minor parties. Dual candidacy means more and stronger competition for electorates, which directly benefits voters).
Napoleon once said "if you want to take Vienna, take Vienna". In this case, its "if you want to weaken parties, weaken parties". Dual candidacy is an inefficient means of doing this. There are far more direct mechanisms to achieve that goal, without incurring the costs of single candidacy. Open lists is one. But the best seems to be simply taking away the power of party bosses to choose candidates. And the way to do that is by a stronger requirement for democratic candidate selection, both for the list and in electorates. Let party members vote, and choose for themselves whether they want yes-men or boat-rockers. Its a democratic solution, which lets voters find the level of independence we want, rather than having someone else's model of how independent an electorate MP "should" be imposed on us.