I was on the vouchers, wasnt in a particularly high BMI category (just over 30 I think so might have been just in the zone to qualify)
I attended every week, (and I did have holiday that I was able to book at the start so didnt lose any weeks), got my Club 10 within the first couple of months, got both sets of vouchers (you have to go for a check up after the first 12 weeks and if you are doing okay, you get the next 12 weeks) and once they ran out, I continued doing SW as a paying member and plan on doing so until I reach target (which is only around 1/2 stone away now).
I agree those stats arent great - but in my case the vouchers were a great help. I would never have dreamed of joining a slimming club before, but now I'm a full convert, recommending to friends, bringing recipes to group, always make stuff for tasters, staying every week for IT etc. I think getting rid of the vouchers would be a big mistake - and I dont agree necessarily it should be means tested - I probably wouldnt have qualified in that case, so probably would never have considered joining, and would be costing the NHS more down the line in my inevitable obesity related illness, diabetes etc.
Those stats do seem rather negative - I volunteer at the pay and weigh so see how many people come in with vouchers, lots of them come every week and have some great losses. I suppose though i probably wouldnt remember those that just turn up for a couple of weeks then drop it, or miss loads of sessions, so maybe its not a good as i think it is.