There is no question about it.

Unpaid internships often serve as a leg-up the career ladder for the well-off. They can, of course, provide valuable experience to young people, experience that can help them clinch the jobs they dearly want, but the opportunity is only available to those who have a separate source of income to sustain them while they are working for nothing.

While some may work a second job, cleaning toilets early in the morning or waiting tables late into the night, others go to the bank of mum and dad. Often, the placements come about as a result of parents having the right social contacts. This phenomenon helps ensure the perpetuation of social and professional elites from one generation to the next and, as such, is to be firmly discouraged.

Organisations advertise for unpaid interns because they can. A poll just over a year ago for the National Union of Students showed that one-fifth of 18 to 24-year-olds had undertaken an unpaid internship, compared to 2% some 30 to 40 years ago. Such is the competition, especially following recessions when the jobs market is depressed, that offering such placements might feel like providing a useful service.

But there is a difference between short-term volunteering and longer term unpaid positions doing work that paid staff would otherwise do. Employing a person for set hours over a prolonged period performing set tasks without pay is widely regarded as a form of exploitation. There are also signs in some sectors that the practice reduces the number of entry-level graduate jobs available. That is why all employers, from wealthy law firms to cash-strapped charities to modestly resourced political parties, should be wary of such placements.

The advertisement made recently for a student to work in the constituency office of the First Minister sets a bad example to employers, especially given the Scottish Government's commitment to boost social mobility. It appears that the role of the intern would be to take on important tasks such as helping with case work and liaising with local press. This could be embarrassing for Alex Salmond, though it is unclear whether he sanctioned or was aware of the advertisement. Nevertheless, he should have confidence that his office is run in a way that is consistent with his principles and beliefs.

No-one wishes to see organisations scared off seeking volunteer help due to draconian policy in this area, and there are cases where internships are legitimate; in counting towards the course requirements of a degree, for instance. It is important to ensure, however, that they do not infringe employment law by requiring interns to do work that officially classes them as workers and entitles them to pay. Labour MSP Kezia Dugdale tabled a motion in 2011 calling on MSPs to guarantee a living wage for anyone employed for a month or more; broadly speaking, a sensible approach.

Many individuals have benefited from unpaid internships but employers of all kinds should approach them with the utmost care to avoid accusations of exploitation.