It's a double edged sword isn't it?
On the one hand unemployed people often lack the skills and experience to get a job. In which case I can see how a scheme like this can help.
On the other hand, right now it's being viewed as a source of cheap labour for the likes of Tesco's rather than a genuine scheme designed to help.
The notion that somehow it's slave labour or that people aren't getting paid for it is simply wrong. You can choose not to do it (and lose your benefits). So it's not slave labour. Also don't forget that most of the time you don't just get JSA alone. There's a whole host of other benefits which most unemployed would be claiming for like Council Tax benefit, child tax credits, housing benefit etc.
The problem I think is with the implementation, not the principle. There's little point in putting people through a week of stacking shelves at Tesco's if they're a graduate who's already got a volunteer position with a view to gaining experience that way. The adviser's need to be given the latitude to make common sense decisions. Or if they have the latitude, to use their discretion.
However, if someone is long term unemployed and are not being realistic in their job search then yes I think this sort of scheme can be helpful. But I personally feel the 'training' should be much longer. Say 6 months and with a much larger focus on education rather than stacking shelves. So customer service training for example. Or for another employer perhaps business administration training.
That way employers have to show more commitment to the training and it's not simply cheap labour and good PR. The employees can prove their worth to the employer over a longer period and have a nicer looking CV.