The former New Zealand First MP is free to cast his vote as he wishes because it has always been an Opposition vote and thus can only increase the Government's majority, not decrease it.
Were Gilmore to likewise become an independent MP, he would be the difference between National having and not having a majority in Parliament on measures where the governing party could not secure the backing of the Maori Party.
Despite coming under huge pressure to resign from Parliament, Gilmore's inclination - at least for now - appears to be to endure the opprobrium being heaped on him by colleagues and remain a National MP and therefore continue voting the party line.
Key's inclination likewise seems to favour keeping the Christchurch list MP on board. While the Prime Minister may have found it difficult to reconcile Gilmore's account of events at the now infamous Heritage hotel dinner a couple of Saturdays ago, he seems resigned to giving his MP a second chance.
Key will judge that the public could not care less what happens to Gilmore. Allowing him to serve penance in private might be preferable to a messy public stoush resulting from trying to expel him from the party.
Key holds all the cards. Gilmore might threaten to vote against National. But that would be an empty threat.
Even if Gilmore was to go independent, it is difficult to envisage someone whose politics are closer to Act than Labour making some overnight conversion to the beliefs of the centre-left and siding with the Opposition parties to spite his former party.
To do so would result in him being treated with pariah status inside and outside Parliament.
His name would be mud. He would be seen as manipulating the proportionality of Parliament and undermining democracy were he to become a traitor to the party on whose coat-tails he re-entered Parliament less than three months ago as Lockwood Smith's replacement.
The pressure on Gilmore to resign from Parliament would become even more unrelenting and unbearable. His actions would almost inevitably trigger calls for the revival of some form of draconian anti-party hopping legislation.
No doubt he has been counselled by colleagues of the inadvisability of going down such a path. He is, anyway, smart enough to work that out for himself.
Chucking Gilmore out of National's caucus and revoking his party membership would do much for Key's standing after his recent stumbles. But would that be worth the potential trouble? Leaving Gilmore where he is, is likely to result in the backbencher disappearing from view as fast as he has shot to unwarranted prominence. Suspension or expulsion from National's ranks would still be held in reserve.
Senior National figures acknowledge Gilmore has done the party few favours by resurrecting the stereotypical image of National MPs as a bunch of plum-accented, know-all Tory toffs and arrogant, born-to-rule Hooray Henrys.
But there is nothing Key could have done about that.
There was also nothing David Shearer could do about the furore surrounding Gilmore affecting media coverage of something of far more significance.
The Labour leader's scene-setting speech on Wednesday before next week's Budget contained no news, but revealed a marked shift in language and the adoption of a much more aggressive tone towards Key. Mixing Alice Cooper with Bing Crosby, the message was Shearer was no longer going to be Mr Nice Guy or Mr In-Between.
Be it is the flesh-searing laser-like stare and acid-filled sarcasm of a Helen Clark, the merciless wit of a David Lange or the evil chuckle of a Rob Muldoon, New Zealanders like their leaders to display what might v be described as "attitude" or "mongrel".
The punishment meted to David Cunliffe late last year was evidence that Shearer is not someone to be crossed. But neither can he easily feign anger in convincing fashion. He comes across as nice - and voters simply equate "nice" with "weak".
Labour's strategists are seeking to dispel that notion by having Shearer tackling Key head-on - something they have been reluctant to do until now as Key's popularity meant such attacks were counter-productive.
Labour now believes there is enough disenchantment with Key amongst those on middle-incomes to warrant a change in tactics, which also involves repositioning Labour and delineating the party more clearly.
Thus has Shearer sought to define the Budget as Key being "more interested in looking after his mates than your mates" and being written for "the boardroom not the smoko room".
Shearer has picked up references made by his Australian and British Labour Party counterparts to "vested interests". It is a tactic right out of Winston Peters' political textbook. You create groups of bogeymen and women - investment bankers, casino operators, property speculators and so forth - and use those stereotypes to force voters to side with you in an "us versus them" scenario.
Labour has already applied that tactic to the electricity price debate by demonising the big generating companies.
In that respect, Labour is delighted only 113,000 or so investors took up the offer of shares in Mighty River Power. National will be relieved that those who did invest saw the share price enjoy a healthy premium yesterday above its $2.50 listing.
Even better for both parties is that the Gilmore problem seems unlikely to rear its head in Budget week - and their plans will not be derailed by the foibles of an irrelevant political neophyte.
