As leaders gathered for the climate summit in New York, I sat in a radio studio in London to help demystify goings-on for different breakfast shows. Hosts had one question for me: “How does Leonardo DiCaprio jetting into New York help save the planet?” The clear implication was that it did not. I found myself leaping to DiCaprio’s defence and, by extension, that of every A-list “ethical” advocate of our times.

Actually, DiCaprio is easy to defend. He’s been banging on about environmental issues for a decade and is a credible climate campaigner, despite the private jets. He is more of an expert than he gave himself credit for in Tuesday’s speech, where he specifically positioned himself as a concerned citizen.

“I pretend for a living, but you do not,” he told those leaders. “Now it’s your turn.” Boom! He is a consummate communicator who can speak without notes and remember all the important bits. And he delivered his speech sporting a man bun (inevitably, this fact was heavily reported).

My confidence in the right celebrity as a powerful advocate has been buoyed by recent strong showings from Angelina Jolie, Emma Watson and Emma Thompson returning from the Arctic. In short, forced to pick, I’m on the pro-fame side of this debate.

Not everybody is so enamoured. UK academic Dan Brockington has written two books on the subject, Celebrity and the Environment (2009) and Celebrity Advocacy and International Development (2014), questioning just what A-listers bring to the party. According to him: “Celebrity advocacy is not as popular as its populist appeal suggests.”

Reading his compelling research, it seems to me that our problem is in quantifying what celebrities can actually do for a campaign. Our expertise is in the “use” phase of celebrity. We know what we can get them to do but we lack data on their efficacy. Brockington (and other academics) point out that celebrity sheen can get in the way of complexity. Conservation is offered as an example: as well as positive pro-nature effects, a conservation programme might result in the removal of an indigenous community. The celebrity would only communicate the positive.

Also, argues Brockington, A-list advocacy makes the rest of us lazy. In effect, we’re using use the celebrity as a proxy for political activism when all they really bring us is escapism (presumably, rather than engaging with the substance of Jolie’s talk, we focus on the shininess of her hair).

Make Poverty History is often cited as a campaign that garnered huge media attention but failed to change much in the long run (apart from making the wearing of wristbands a massive trend). Then there are the undeniable links between self-interest, celebrity and consumer culture. Victoria Beckham, who, in my opinion, still has a bit of work to do as an advocate, could have been said to have reached the apogee of this on Wednesday, when she let it be known she was missing the opening of her London store to address the UN, where she will work with the organisation’s HIV and Aids charity. What sacrifice!

In The Ironic Spectator: Solidarity in the Age of Post-Humanitaranism (2012), academic Lilie Chouliaraki ran a comparative analysis of Audrey Hepburn’s work for Unicef versus Jolie’s for UNHCR (UN High Commissioner for Refugees), a sort of celebrity death match for advocacy. Through the lens of this analysis, Jolie didn’t seem to contribute as much as Hepburn, as Jolie was mainly inclined to talk about herself and her experiences.

Is this fair? Times have changed and so has celebrity. Gone is the stiff upper lip approach of Hepburn’s day, supplanted by personal confessions. I found Watson referring to her experiences as a young girl represented the most resonant part of her HeforShe speech.

Overall, the content is becoming much stronger, too, and celebrity advocates realise they are there for the long haul and have to take a certain amount of responsibility for a campaign; two years after Joanna Lumley spoke up for the Gurkhas she had to defend criticism over the long-term impact.

But I appreciate there some points where all this celebritisation sticks in the craw. I truly do. The UN seems to be giving out ambassadorships like confetti at the moment; if Kim Kardashian gets one, I might need to change my views.

There are also times when using celebrity faces seems counterintuitive. Fairtrade, for example, is all about the producer and the equitable supply chain. They are supposed to be the stars of the show, so when a celebrity face becomes the focus, some long-term supporters are bound to get upset. But this is a luxurious, utopian gripe. Gone are the days when Fairtrade was about “biscuits for vicars” – now it has to shift product.

In common with many charitable initiatives, Fairtrade is also under pressure to attract a younger demographic. Presumably this was why the charity took Cheska Hull of Made in Chelsea fame to Ghana for a Hello! spread. For her efforts, she was uncharitably given a kicking on social media.

Poor Cheska. In truth, academics and activists are not that keen on pop celebrity culture. By Brockington’s own rather sweet admission, he was well through the final edit for his first book before he realised that Brangelina was not a single person. Elsewhere, much of the anti-fame analysis has been conducted by Marxist academics who only see an unholy alliance between celebrity culture and NGOs and corporations.

Critics often miss the heavily nuanced relationship between NGOs and “talent”. It is its own ecosystem, if you like. The celebrities are not interchangeable or incidental: Joey Essex (The Only Way is Essex) isn’t going to stand in for DiCaprio at climate talks. In any case, the lines are becoming blurred: some of the key analysts and politicians, particularly around climate change, are also well known. Mary Robinson, former president of Ireland, is the UN’s envoy on climate change.

There are good times to use celebrities, and bad. We also have to manage our expectations: it’s true that celebrity blogging probably won’t lead to global-level system change as quickly as people throwing bricks in the street but it has a place. Jolie, Watson and DiCaprio have raised the bar on celebrity advocacy. Let’s give them some respect.