You are trading your personal privacy for a few bucks. More power to you. The problem is that few people understand the ramifications of giving away their privacy until it bites them in the sass, at which point it is too late to get your privacy back.
One example - toll road transponders like EzPass. Much easier than paying cash - and frequently cheaper too. The problem is that all the records of your ezpass use end up as a permanent record in their database. Those records have turned out to be a gold mine for divorce attorneys looking to prove infidelity (civil cases only require a "preponderance of evidence" not "beyond a reasonable doubt"). Saved a few bucks up front but those people ended up losing 1000x what they saved in their divorce settlements.
Not the same as filling out surveys and scanning in all your purchases - but it isn't difficult to see scenarios where that becomes a problem too. Lets say you smoke and you fill out a survey about how many packs a day you smoke - maybe sometime in the future your health insurance decides to decline coverage to people with a history of smoking as much as you admitted to in that survey. Or maybe it isn't something so obvious - say you don't smoke but you eat a lot of junk food and in 10 years time the war on junk food has made it politically correct to treat junk food eaters as badly as smokers and you can't get a job because you have a record of eating too much junk food (just like a few hopsitals now refuse to hire smokers).
You can't know the future - giving away your privacy for a few bucks is like opening pandora's box. You have no idea what you are letting loose with your privacy but once it is out there in the hands of amoral corps and their government lapdog you'll never ever be able to put it back in the box again.
For me, the trade-off of a few hundred bucks in hand against who knows what kafkaseque future is not even close to being worth it.