Anyone who uses that argument, ask them to hand you their phone, unlocked, and a list of passwords for all of their accounts. If you have nothing to hide, why would you have a problem with me poking around in the most intimate aspects of your life?
This doesn't really convince them. To them, a friend might be someone they want to hide things from, but they won’t care about some unknown government entity having access.
This comes from not knowing what organizations are likely to do with it.
Would they be okay with the data being used to overcharge them whenever they're in a hurry because the organization knows when they don't have time to comparison shop?
How about maximizing their tax burden by using the data to calculate the highest tax rates each area would tolerate before moving or changing their vote?
Suppose the government falls under the control of the party they don't like. Should they have access to the data that allows them to most effectively target their propaganda?
And it doesn't even have to be an organization. How often do you hear of cops or civil employees poking around in records they have no business to poke around in?
But I find the sort of people who believe in "nothing to hide" tend to believe in the efficacy of bureaucratic systems set up to prevent that sort of thing. A belief (ironically) bolstered by the privacy those organizations secure for themselves, so that those kinds of individual abuses are rarely discovered. Who watches the watchers?
Whereas it's easy to understand that if you don't have a high opinion of the opposing party's most recent President, you might very well not want their administration knowing everything about everyone.
The two arguments aren't mutually exclusive. The shock value of 'let me see your phone then' can lead into the greater discussion that AnthonyMouse is presenting. The emotional hook can often be just as useful as the logical argument, especially when dealing with someone who didn't really reason themselves into a given position in the first place.
Unfortunately, it's an issue that's hard for many to understand until it directly affects them. Sometimes you need to find something relatively innocuous but still embarrassing and 'give them a thwap' with it, but even then the chances of them understanding are still low.
