OK, so there is no top up of their benefits. So, in effect, they are working for their benfits, and gaining the rest of the stuff I mentioned at the same time. The benefits can be looked at as wages in these circumstances.
Lets look at it this way…
Two neighbours live in houses which have the same running costs and have the exact same circumstances. They each recieve identical benefits, which allow them to live a basic lifestyle (surviving I believe you called it) and they are both actively seaking work, since leaving the same job, which they both worked at the same company since leaving the same school with the same qualifications. They are both healthy and have no reason not to work (disabilities etc…).
Neighbour one (lets call him Andy) decides to go and do the work scheme with Tesco’s, and works 8 weeks, at 40 hours a week, and each evening comes home to the same house, has his dinner and goes to sleep ready to go back in to work the following day.
Neighbour two (lets call him Dave) decides not to go on the scheme, and instead decides to sit at home most days, looking for job adverts and generally living his usual lifestyle in the hope that one of the jobs can pay him enough to get off his weekly dole, and get him back onto the work ladder without being worse off through losing his other benefits (this would be through the usual thing where the worker then loses certain benefits because he isnow deemed to be earning enough that he doesn’t need the extra help with his rent etc… This is usually not the case!).
8 weeks down the line and Andy has finished his work placement. He returns home to his house and carries on looking for work. He has made a lot of new friends/acquaintances and has added a number of new skills to his CV (it’s not all shelf stacking!), including his Health and Safety at work First Aid course, an NVQ in customer support and possibly a few others (remember, it’s not all about the shelf stacking!) and is generally more upbeat about life and looking forward to getting back into a work environment.
Next door, Dave is still looking for work (same as Andy) and has spent the last 8 weeks attending his job centre and going to job inteviews, to no avail. He is still jobless and still claiming his benefits (same as Andy).
So, at the conclusion of the eight weeks, they are both still living in the same houses, claiming the same benefits and living the same lifestyle. They both go into the job centre and apply for the same job. They both go for the interview… YOU ARE THE INTERVIEWER!
So, firstly, who is more employable?
Secondly, after the eight week period who is generally better off out of the two? More to the point, who has lost out?
Seems to me that the only major gripe that you would have here is surely against the supermarket who you seem to think are exploiting the individuals? What if there was the chance that some of these people were offered jobs on the back of the work experience?
Speaking of work experience, do you think this is the first time this has been tried? I remember as a 16 year old, before I got my apprenticeship, I was doing work placements as part of a jobcentre scheme. I was paid the princely sum of £35 a week, and worked in all sorts of places, usually for a few days or so, which included a dairy where I had to stand in front of a production line and check to see if any of the bottles were broken or cracked before they went into the next area to be filed with milk. Literally the most boring job I could imagine doing, but it gave me a good work ethic. I felt like I had earned the money that popped through my door on the job centres cheque every fortnight! 
