To be honest there are some free products out there that are just as good in my opinion, although a paid for version may offer more options.
A bit like people use AVG Free Antivirus programme as a suitable alternative to paid products like Norton, people also use Ad-aware as an alternative to paid products....and with arguably just as good results.
If its a tiny price to pay and your happy with the product then maybe it would be worth the cost for spysweeper, depends how happy you are with the product...if its pricey then Ad-Aware is just as good a product to have. Besides why not try a free one like this until your product is due for renewal/purchase and then decide.