The fortunes of Michelle Obama and Valérie Trierweiler, arguably the world's most high-profile first ladies, contrasted sharply last weekend. While the grandly abbreviated Flotus (first lady of the United States) celebrated her 50th birthday with a White House party, France's de facto première dame checked out of hospital. She had spent eight nights under medical supervision after learning that her partner, President François Hollande, was having an affair with an actor.

The relative attractiveness of any job is best measured when times are bad, and right now times are very bad indeed for Trierweiler. As Michelle Obama enjoys fluffy tributes from the great and good of American society for her support of Barack Obama, her French counterpart is effectively pleading to stay with Hollande.

Aides to the Socialist president have indicated that he wants to make his lover Julie Gayet the new first lady. Hollande said he will "clarify" his position before a trip to Washington DC to stay with the Obamas in February. He spent just half an hour with Trierweiler during her entire stay at the Pitié-Salpêtrière hospital in Paris, and is said to be "managing" her unhappiness as a political, rather than emotional, problem.

Beyond Hollande's capacity for cruelty, the crisis in the French presidency says everything about the abject feebleness of the first lady concept. At a time when a glowing media image has never been more important for world leaders, many of their unelected consorts still knock about the corridors of power, and indeed the international stage, with an alarming lack of direction.

Even Michelle Obama, who trained as a lawyer, had no hope of practising on taking up the role. Now her main job is as "hostess of the White House", allowing her to invite stars such as Beyoncé to her bashes, as she did last Saturday. Yes, Flotus has an office and a press secretary, but she has no salary and the majority of her tasks are decidedly shallow. As throughout history, first ladies around the world are largely required to be presentable escorts when called upon, and to make their husbands look good.

Michelle Obama has tried to make a difference, mainly through campaigns about obesity and other social ills. But it is as a winner of "best dressed" and "most inspiring" awards that she remains well-known. In this sense, popular perceptions of what a modern first lady does are sexist and trite. Michelle dances, she sings, she cries in appropriate situations and she is a close confidante of Oprah Winfrey. Thus highly educated, talented women such as her are effectively told to suspend their careers to become state-sponsored ladies who lunch.

David Cameron's wife, Samantha, is not an official first lady – British heads of state are royals – but when her husband became prime minister, she left her job as a creative designer to adopt part-time roles, mainly for charities and fashion organisations. As far as Mrs Cameron's potential influence as a dynamic prime ministerial partner is concerned, forget it. It is getting to the stage where people do not even know what her voice sounds like. The once much-vaunted "Sam Cam" brand has never taken off, leaving Mrs Cameron as a bizarrely hollow public figure.

In this respect, she has become a female version of German chancellor Angela Merkel's husband, the scientist Joachim Sauer, who is known as the "phantom" because of his ghostly lack of presence (on the subject of pervading sexism, it is noticeable that Sauer is never referred to as the "first man"). He stayed at home to watch Merkel's inauguration on TV. Because of his gender, he is not expected to entertain or smile sweetly. Germany's actual first lady, presidential partner Daniela Schadt, is by no means a household name either.

Schadt is, like Trierweiler, unmarried. While many social conservatives point to this single status as being a huge disadvantage in itself, in France it merely highlights the pervading misogyny of the political establishment. Gallic leaders since Napoleon have traditionally kept lovers hovering in antechambers, to the extent that they are interchangeable with spouses. François Mitterrand, France's most famously monarchical socialist president, kept a family hidden at the taxpayers' expense.

Even now, Hollande is using disingenuous references to "privacy" to cover up what looks like a callous treatment of his girlfriends. These have included not just Trierwiler but also Ségolène Royal, the mother of his four children who was dumped while running to become head of state herself in 2007. Gayet will certainly not come out of the quasi-feudal presidential courting system unscathed either.

Trierweiler always claimed that she would not become a presidential "wallflower". If, as expected, she is kicked out of the Élysée in the coming days, she will get no compensation. The role of première dame comes with five clerical staff costing around £17,000 a month, but everything else is down to the bon vouloir of the president. Pointedly, Hollande stressed last week that the role was an "unofficial one" with "more to do with tradition" than anything else. Senior colleagues even called for it to be scrapped.

First ladies have no financial security, nor guaranteed tenure. Their ill-defined, awkward job may be temporarily perk-rich, but it ultimately leaves the incumbent in a fragile position. Trierweiler's slow exit from presidential life has been brutally humiliating. The sole consolation for a female journalist who has held on to her job at Paris Match might be a tell-all autobiography. It will be grim, but it will at least make clear that the job of ex-first lady is invariably more fulfilling and lucrative than the real thing.