About 15 years ago, when Alan Clark was still alive and when he was Britain's only entertaining MP, I rang him up and asked if I could interview him.

He said he'd be delighted, but I would need to pay him for his time. Oh no, I said, all prissy and shocked. The Financial Times would never consider such a thing. In that case, he replied, no dice. Saltwood Castle, his medieval family home in Kent, needed a new roof, and there was no way he was going to work for nothing.

At the time I took this as evidence of Clark’s solipsism and greed. But now I have changed my mind.

For him to ask for money was so reasonable there was no need for him to invoke the leaking roof. He was selling his time and his opinions, and he had the same right to charge for them as someone selling soap powder.

I'm not suggesting that everyone who is interviewed by this newspaper should immediately slap in a bill. It is clearly such a great honour to be featured in the FT that no payment is needed. Instead I am talking more generally about all the things people routinely and increasingly do for nothing, flouting the labour supply curve that says that when no wage is offered, no labour ought to be supplied.

There are, of course, the interns who slog away for no payment. This system is exploitative, discriminates against those who don’t have rich parents and is often illegal; but even so, it isn’t altogether senseless from the intern’s point of view – they gain experience and doors may open.

More of a mystery is the explosion in the unpaid work done by professional people with lots of experience and with satisfying day jobs but who still insist on filling their spare time with extra work for which they are paid zilch. They blog and tweet for nothing. They talk on panels, go to conferences, give advice and even write books – all for nothing. But why?

With Clark as my role model, I have taken to refusing all such things. Would I like to go and give a talk to students at Oxford? No, thank you. Would I like to talk on a panel about corporate governance? No, I wouldn't. Would I like to write a blog for the Huffington Post? Certainly not.

To all these invitations I explain that I don’t approve of working unpaid, and invariably I get the same response I gave Alan Clark all those years ago. How greedy and selfish, I can hear them thinking as they bustle off to find someone happier to oblige.

I can think of only three situations in which it makes sense for professionals to work for nothing.

The first is when it’s for a good cause. But then it’s voluntary work and the whole point is that you don’t get paid.

The second is if the work is truly fascinating or is something you’ve always wanted to do but couldn’t do otherwise. Recently I was asked to talk on a panel in a West End theatre, and as it’s not looking likely that I’m ever going to make it as an actress, I snatched at my only chance to tread the boards on St Martin’s Lane.

The third reason is that it's good publicity. This is why most people work for nothing – they think it will help them sell books, or build their brands, or be good for networking. I can see that if you are trying to be better known and you get invited on Oprah, then you must go along with it. Or if you are asked to be interviewed in the FT – that, as I've already pointed out, is gold dust.

But many things people do to help sell books or sell themselves are not obviously effective at all. In this age of big data, it ought to be possible to calculate precisely what works and what doesn't. Most of the bloggers on HuffPo seem to have almost no comments, so one can't see them shifting many books as a result.

There is one more reason people agree to unpaid work: because it feels good to be in demand. But this is irrational, as to value yourself at nothing should make you feel very bad indeed.

If we all did a Clark and refused most unpaid work, I predict it would lead to a rise in happiness-adjusted GNP. There would be far fewer pointless events, which would mean everyone could go to the pub or see their children rather than sit through an evening event on corporate governance.

Moreover, the quality of output would rise. Money isn’t perfect, but it is the best way we have of rationing effort. If you are paying someone to do something and it’s no good, then you can tell them to do it better.

And finally it would mean that those old-fashioned organisations that still pay people a salary in return for labour would get better value, as people would stop spending their lives moonlighting and get on with what they were paid for. – Copyright The Financial Times Limited 2013