I think the purpose of the 'free foods' is mainly psychological. It's been demonstrated time and again that if you say to people 'you can't have that' they immediately want to eat as much of it as they can. This is why many conventional diets fail.
Research has shown though that the reverse is also true. If you tell people they can have as much of something as they want, they don't tend to binge as you might expect, they tend to eat until they're satisfied and then stop. It's this research that influenced the way SW was set up.
I think it's genius. I'm convinced it was the concept of free foods that got me out of a chronic 20-year bingeing habit. That's an example of what a wonderful bit of psychology it is.
It's not a 'magic formula' that will make you lose weight even if you eat ridiculous portions, though some people seem to treat it as such. Ultimately, to lose weight you need to take in less calories than your body is using up. Your body doesn't say 'oh, this is SLIMMING WORLD FOOD, I won't bother counting that as energy, it's FREE!!'
Look at it this way: to gain 1lb of fat per week, you only need to consume an extra 500 calories a day over and above what you need. To put that in perspective, that's a small portion of rice or pasta, a jacket potato with a small tin of beans, a sandwich .... it's really not much at all. Your body will gain weight from that extra 500 calories whether it comes from SW 'free foods' or a big bowl of ice-cream. Your body doesn't differentiate between them, it just thinks 'oh good, 500 extra calories, I'll lay that down as fat'.
It's down to us to teach ourselves portion control - to take this liberating concept of 'free foods' and use it wisely. If we do, we'll lose weight and keep it off. The SW plan is such a fantastic opportunity to learn to eat healthily and sensibly for life. But only if we use it well.