One of the major reasons I've heard about, and from my own government no less as a warning (when facebook was new) is none other than preventing crime from being committed to you in the first place. Lets spin things around, government surveillance is bad, sure but government being able to watch you means that anybody else with a relatively simple set of tech skills can tap in to the government's surveillance tools (like facebook) and abuse it to find the same information the government would find simply by looking around, they don't even need to be hackers...
And what can they, what can criminals do with your now easily accessible information?
They can find out where you live, find out if you have a security system at home or not (based on pictures of your home) they can find out when you're at home, and when you're not, they can find out who lives there with you and discover all the same stuff about them, then based on that they can target your home when you're not there (raid the place), or when one member of your family they want to target is home alone (sexual predator? someone in the mood for a revenge killing?) and act based on what they have found.
Assassins and the like have in the past to be successful needed to get in close proximity of their targets to spy on them, no more, all they need to know is more often than not on facebook, no espionage needed. This will be a particularly big problem if you're a public figure (notice how they don't participate in social media the same way us normal folks do? Well of course not, they can't! because it will paint a target right between their eyes for anyone who wants to do them anything)
But all in all, don't you just think it's pretty damn creepy that anyone anywhere can know anything and everything there is to know about you without you even knowing they exist in the first place? Does that feel right? It certainly does not feel right to me, actually kinda scares me even if I don't have anything to hide by definition, I'm not doing anything illegal...
Or am I? The legal systems everywhere in the world are such a clusterfuck that millions of people are unwittingly commiting crimes, here are a few examples:
Stuff we may consider bad habits, don't realize we're doing, or do but just have no idea it's illegal, under constant surveillance we may or may not be arrested for these things, but one of these days someone somewhere will have it out for you and the data about these illegal things you didn't know anything about can be used against you in a lawsuit or to deface your public image if you care about that. "This musician downloaded a song once!", "Oh what a shithead, since he thinks it's ok lets all pirate his shit!" or "Oh what a shithead, let's boycott his ass!"
Many people even cheat on the tax because they can't afford to pay it (out of need) they may feel ashamed of it, but they are in need of getting away with it. These can be regular everyday normal folks, and while I'm not trying to protect them, you never know when this might be you or your family. When they do it they may feel embarassed about it, know it's illegal, but are simply backed into a corner and feel they have no choice. Without privacy it's true, they don't have that choice.
We also behave differently when we are under surveillance, and I quite liked this video which delves a bit into that one. https://www.youtube.com/watch?v=AW7aU3zv-4M
see also the government perspective: http://www.bbc.com/future/story/20140209-being-watched-why-thats-good
A sane person's perspective: http://www.alternet.org/news-amp-politics/surveillance-thinking-and-behavior
As the common saying goes: "The devil is in the details", don't let yours be too easy to find.
And as for why security is important, you simply can't have privacy without it. You can post private data on facebook, anyone can read it, you can post private data on private channels on facebook, most who are determined will find a way to read it. You can send someone a private email, there are many ways to read that.
If end to end encryption is used, the ways in which that private email can be read is severely reduced. If hard drive encryption is used, the ways in which your data could be accessed when you are not at home or if someone lets say steals your computer (it happens, it does, happened to me, my desktop was stolen from my home... I'm just a normal dude...) all that data is accessible to anyone who gets their hands on your computer, unless your drive is encrypted. And well, so on.
You can think about it like this. Two people in the middle of the street chatting privately, anyone can listen in. Two people in a sound isolated room, it suddenly gets much harder to listen in (the only way is having a bug planted inside the room). The former is privacy without security, the latter is privacy with it. It's not that security makes it impossible to spy on you, it's that it makes it too hard to be worth the trouble for whoever might.