But how often is a Electorate MP really tossed out by an electorate. More often a few hundred or thousand swing voters change their mind and an MP loses their seat. A case in point is Waimakariri. Clayton Cosgrave won Waimakariri in 2008 with 16,360 votes to Kate Wilkinson's 15,970. In 2011 Kate Wilkinson won with 16,787 votes to Cosgrove's 16,145. Did the voters in Waimakariri really toss Clayton Cosgrove out considering he only lost 200 votes?
What really happened is that ACT decided not to field a candidate, with their 1700 votes in 2008 going to Kate Wilkinson in 2011. If the Green Party had chosen not to run a candidate then it is most likely their 1200 votes would have gone to Clayton Cosgrove and he would still be the Waimakariri MP (A much better solution would be preferential voting, which I will touch on in a later post).
A few hundred or thousand voters changing their minds does not mean that an MP has nothing to offer his party and the people of New Zealand, they can also still act as a secondary electorate MP.
Removing dual candidacy would also unfairly hurt smaller parties who have no chance of winning the majority of electorate seats dominated by Labour and National. Denying them an important forum to promote their ideas and limit their media coverage.
There is always the possibility of a sitting electorate MP being soundly rejected by their electorate and returned to parliament on the list, however democracy will never be perfect and removing dual candidacy will be of more detriment than the occasional back door MP.
Dimpost again provides a good analysis on this issue:
I tend to think this is a complaint made by people who just don’t like MMP, rather than a valid problem with the system.