The practice has its critics. Union leaders are decidedly edgy about free labour competing for jobs with their members. There are charges of exploitation and bad treatment. The caricature has unpaid interns working photocopiers or being sent to collect sandwiches or laundry for management, without gaining any useful hands-on experience.
Some critics detect class bias in unpaid work experience, saying that only affluent middle class children can afford to work without pay, and that the middle classes network with friends to manoeuvre their children into suitable positions with key firms.
Labour leadership candidate, Ed Balls, supports the proposal that all interns should be paid the minimum wage to prevent exploitation, though it is unclear whether this applies to his own staff and campaign workers.
Finding work experience takes effort. Many City and media organisations are inundated with requests and have a gruelling process to select suitable applicants. It is very competitive, with the biggest firms able to pick and choose the best applicants.
For employers, work experience is a useful way to assess a candidate's strengths and weaknesses, and to make a full-time hiring less risky. Over 1 in 5 of the 2009 graduates in employment had previously done work experience for the firm that hired them. And it works the other way, giving interns the chance to see whether or not the firm they have chosen is really the right one for them.
Work experience wouldn't work if minimum wages were required. It costs employers time and effort to train and oversee interns, and it takes time before their contribution justifies the effort it takes to teach them.
The truth is that young people look further ahead than their predecessors did. Instead of earning money by picking fruit and only going into a job after graduation, most are now sounding out possibilities ahead of time. They use vacations and post-graduation time to experience the work environment and to enhance their own employability by stacking up experiences and qualifications.
Undoubtedly, some only want to put a line on their CV and contribute little to the organisation that has taken them on. And doubtless some employers make little effort to train and prepare their interns. But the experience has become widespread precisely because it is usually positive on both sides. People are better prepared for work than they were, and they go into jobs more aware of what work entails.
This has developed without being driven or directed. Those who talk of exploitation and class bias are missing the point. It is widespread because nearly everyone benefits from it.
Madsen Pirie is President of the Adam Smith Institute