What do you mean "how do you tell someone I don't want to follow your tweets, status, myspace, whatever they are called"? I just don't follow them, plain and simple and they don't appear in your view.
Why do you have to explicitly explain to them why you don't follow them? It's your personal choice and you don't have to explain yourself to them. If you consider you have to then that's a different issue and you can always resort to the good old "Your updates do not hold any relevant content for me".
As for you not seeing the benefits that's a different part because when you share you don't usually do it expecting something (a benefit) in return; I share an interesting link I find on Twitter because I believe that other people who follow me will find it interesting, I put up a question there because I know I'll get an answer faster than if I would post in on a forum and it'll reach a larger audience than if I would just send it in an email to a friend or coworker.
Nobody says you have to post an update saying "PFC XYZ died today"; you can very well post an update saying "Got to see this great new piece of tech in action. Recoil on it is awesome" or anything you think will be valuable to others that follow you.
It's all about finding value and spreading it. The crap is there but for every crappy tweet there is at least one that has something interesting in it.
As for using IM versus Twitter/Facebook to stay in touch, they're very different systems due to their nature: one is real-time, the other is near-real time.
I may not be online when a friend back home is in order for him to tell me that he wrecked his car or failed an exam but I can sure as hell be able to see his tweet about it when I open my client and send a reply even if we're not both online.
And regarding that soldier that posted that information: it's up to your organization to set and enforce it's privacy rules. You can't blame a service for allowing anyone to post anything but you can blame the person for posting that crap despite knowing that it contravenes the security policies.
For example, in the case of my workplace, we have a very clear disclosure guideline which states what can and can not be released to the general public. We manage a hell of a lot of databases and server clusters and we are allowed for example to tweet that "Yeah, a database is currently down and we're working on it" but we may or may not be allowed to say "It's down because there was a coolant leak that lead to the overheating of the rack and blowing a couple of drives". I might get that as a DM on Twitter because I'm an employee and DBA and need to know what happened so I can fix and work around the limitation (and we use Twitter DMs because it's a free way to send text alerts: all the team is following a master account and has mobile updates turned on and as soon as a tweet is posted on that account everyone is notified).
If I was to release that information to the general public I'd get my ass fired in a second and in a huge fucking lawsuit over disclosing sensitive information.