Had it relied on the Maori Party vote to govern, there would have been no asset sales, limited welfare reform, and few labour law changes.
A Government in which he required the Maori Party to be onside for every measure, rather than simply confidence and supply, would be a very different kettle of fish and quite likely unworkable.
Asked this week whether he believed he could govern effectively if he required Maori Party support for every vote, Key answered that he believed he could. That is rather optimistic.
The Maori Party is fundamentally a left-wing party. It opposes about three-quarters of National's legislation. It can justify its current deal by saying it gets enough without having to compromise too much. Even that limited support has been damaging for it. Becoming the only crutch by which National manages to hold on to power is likely to kill it off.
That is why NZ First is more attractive to Key than he is letting on. Take the personalities out of the equation and a National-NZ First Government would be far less restrictive for National than one relying on the Maori Party.
There are the obvious points of difference, but there are also major points of difference between Labour and NZ First. It will help that National's asset sales programme will effectively be completed by the election, and Key has said he does not expect to go to the polls campaigning for more.
Financially, it might be wiser to hold off on the Genesis float for a while - but it would also be politically inconvenient. National really has only a one-term window for that policy, especially if it needs NZ First.
However, personalities are involved. Key and Peters cannot resist baiting each other. The latest example was on TV3's Firstline. Asked about ministerial portfolios for Peters, Key said foreign affairs might not be an option this time round because "Winston may be at a different point in his political career. Being Foreign Minister is tremendously hard".
The clear inference was that Peters had "aged off", to borrow the GCSB's terminology.
Peters' Achilles' heel is his touch of vanity. Effectively calling him old and tired is a strange tough-love approach to take to coalition building.
When Peters was told of Key's comments, he laughed and retorted with "so who's losing his hair?" - a dig at Key's receding hairline compared with his own crowning glory.
At first glance, the antipathy is deep enough to say a National-NZ First arrangement will never happen if there is any other alternative. However, there are benefits in it that Peters will be mulling over. More ministerial posts comes to mind. Peters may believe it is preferable to play second fiddle in a National Government than third fiddle in a Labour-Green Government.
There is also a message Peters cannot ignore in NZ First's 1.5 point jump above 5 per cent in the latest Reid Research 3News poll. It came straight after Key ruled NZ First in as a possibility. Key ruling Peters out in 2008 was seen as a critical factor in NZ First's failure to get above 5 per cent in 2008.
A further poll question asked whether Key was right to rule Peters in again and 80 per cent of NZ First voters approved. The timing of that poll lift indicates much of the extra support came from traditional conservative NZ First voters who lean towards National rather than Labour. Those voters used to be Peters' core base. He lost them once, he cannot afford to do so again, especially if he hopes his party will survive when he does age off.