It's absolute garbage.
To begin with, everyone has something they don't want the whole world seeing.
More importantly, what you have that needs hiding can change overnight.
While bringing up ww2 is bad form I guess, have you ever looked into how the Nazis found all the jews living in the Netherlands?
The dutch government kept some of the most detailed and accurate census data of that era.
And while the dutch government could certainly be trusted with it, and wasn't doing anything suspicious, guess what? Germany invaded and took all the records, and used them for horrific purposes.
Ask yourself, next time you get a census form... If you had stated you were jewish in the 1920's, that would have been completely innocent. Yet having said so in the 20's or 30's would likely have gotten you killed in the 1940's...
People that say 'I have nothing to hide' are probably lying anyway, even in the immediate sense, but they are also being incredibly short-sighted.
Hopefully it won't but events like what happened in the second world war could certainly repeat themselves. Misuse of confidential information by a group you didn't even give it to in the first place...
It probably won't be jews this time though. You never can be quite sure what would get you in trouble...
Besides which, if some organisation collects massive amounts of data on you, they may start putting things together out of context, and come to dangerous conclusions.
2 + 2 + 2 = 28
Can happen really easily.
Let's say I did an internet search for household cleaning products, read a wikipedia article about gunpowder (for historical curiosity), am registered with a flying club, and have searched some stuff related to islamic extremists (just because it's been in the news a lot), and maybe a few sites talking about the evils of capitalism...
What do you think someone might infer from that? How likely do you think it is they'd reach some problematic conclusions?
Even though there's no pattern there, and in reality it's a series of random things, it's easy to assume it's all related, and means something that would make a person look suspicious, even though they are not...
That's to say nothing of other inferences...
Oh wait, this person is an adult, and they like anime... They must be a peadophile... >_>
You know. What seems innocent from one perspective can look suspicious from another.
And you really can't predict it that easily...