While I agree with the weighing usefulness bit, there are issues with preserving your privacy.

One is that your ISP has access to all of your activity. Try to circumvent them by using a VPN, and the provider of the VPN knows about all of your activity. That pretty much leaves you with technologies like Tor. Even if you trust the network itself, it creates a huge bottleneck (at least from my experience)

The second issue are social or employment expectations. While you do have a degree of choice in this domain, the choice (more often than not) severely limits your options. Whether you like it or not, your employer may be outsourcing that corporate email account or website to Google. Friends and family expect to keep in contact via Facebook. A community organization may communicate to its members via Twitter. Yes, you can find ways to avoid all of that stuff. There is a huge cost of cutting yourself off from others.

Technical knowledge is also a huge hurdle. A lot of people aren't aware of the privacy implications of various technologies because they weren't brought up to think about them, or they do think about them but don't understand how to use the technology to offer a degree of protection of their privacy without cutting themselves off, or they do think about their privacy but they have far larger priorities in their life. Perhaps all of this sounds ignorant to you, since a lot of technical people seem to have that attitude, but the thing that we must realize is that many of us (on MR) have prioritize technology in our lives. Those other people have other priorities and, in many cases, they are just as important as ours.

The list can go on. For example: how do you deal with people who may unwittingly disclose information about you. You may use a secure email service, but they may be using something like Gmail. Anything that they say to you and anything that you say to them is automatically at Google's disposal. (Even if neither party uses Gmail, or similar data mining services, most email is still sent as plain text. That isn't going to change any time soon.) Or how do you deal with the inadvertent "slip-up" on your part? Clear all of the data from your web browser? Been there, done that. It turns out that browsers store a tremendous amount of data that has to be rebuilt.

So no, protecting your privacy isn't a simple decision.