i would say no, unless there are lots of things you usually buy that are discounted with the vouchers. i wouldn't use it to get 'extras' if you are tight for cash because things are cheaper at asda overall.
It's only (I think) 1 point for every £4 but by the time you have your double points and then the extra for using the credit card it adds up.
Like I said before though we have spent thousands on the card. We pay for holidays, cars, large items, building materials on the card and then pay it back straight away so we have accumalted a lot of points.
Oh goodness don't use your vouchers as payment in store. There if you have a £10 voucher you get £10 knocked off your shopping bill. Use it as a reward such as the hotels or other offers. £10 is then worth £30!!