Shane's heart wasn't in it
Shane Jones's decision to leave politics was made, according to the man himself, on the grounds that his heart wasn't in the job. And perhaps the public are better off if any other parliamentarian whose heart is not in the job would follow Jones's example. So good on him for being honest. Moreover, he clearly would not have been comfortable working with the Greens, so any future Labour–Green coalition would have been difficult with him in it. On the other hand, he had the ability to reach out to Winston Peters, in the case that Labour would have to do a deal with NZ First.
Unfortunately for Labour, though, Jones's sudden departure sends negative signals, even if Jones never intended it that way. First, it comes across as a vote of no confidence in Labour by Jones. And it risks alienating some of the middle-of-the-road male voters that Jones was able to appeal to. The resignation lances a boil for Labour, from one point of view, but, from another, it shows up all too clearly where the infection lies: that is, in the ideological split within Labour between pro-growth centrism and a more Green-friendly alternative-progressive direction.
Labour is not only losing a politician who was popular with many on the left (and unpopular with many too, it must be admitted). Jones's decision to leave – following a job offer from a National minister – will be seen by many voters as 'jumping from the sinking ship.' Shane Jones has done National a big favour.
The timing was ironic too: just as Labour were going after Judith Collins, the Nats were quietly white-anting Labour's front-bench and wooing Jones away.