I must say I am hesitant to write this response; I know that I can come across as pedantic at times, and that's really not what I want to do here.
Therefore, I will limit my comments and point readers in the direction of previous pieces I wrote concerning both the use of Twitter in class (although with my updated hashtags work, it's definitely time to update that post!) as well as student responses to using Twitter in class.
I do ask forgiveness from the original blogger for taking so much of the following chunk of text, but it's impossible to comment here in a way that would be most effective without actually presenting the example as originally posted.
Sometimes I think that folks dream of the following as being the kinds of conversations that students would have using twitter:
S1: In Bio listening to a great lecture on cell division. She ROCKS!
S2: Reading "Chapter 7" for Mr Wilson. Not my favorite so far but I'll get through it
S3: Great quote from Mr B: "History is written by the winners." He said it's not his original quote, tho'
S4: @S1 Mrs D is awesome! Looking forward to 5th period when I'll hear it, too
S4: @S2 Tell me about it. I've got to read it tonight. But I'm a fast reader.
S2: Anyone know a good website to help me to understand Chapter 7?
S1: @S3 Yeah, I heard that quote before, too. Hold on and I'll look it up for you.
Now, THIS is what would PROBABLY happen if students used twitter:
S1: wht's 4 lunch? I'm staving!
S2: NFW I'm readg that @#$ Chapter 7. Hes got 2 B Sh*ttin' me!
S3: "History is writn by th winners?" DUH!
S4: I get out of her class 5th period, dude! S*cks 2 B U!
S4: @S1 its only 9:30 dude! U cnt B hungry alrdy U pig!
S2: Any1 know if any of ths @#$% in Ch 7 will B on the test?
S1: @S3 No duh! Who ELSE wld write it. Hes so lame!
Actually, neither example has anything to do with the way we use Twitter in class.
While conversation is one use of Twitter, there are plenty of others.
As I've described before, my students use their feeds as lifelines on quizzes and tests, as a way to share links and organize collaborative assignments, as a tool for hashtagging collaborative reference bibliographies, and as a collaborative note-taking backchannel during lecture and class discussion. We also use Twitterfall as a discussion starter and I project our Twitter feed on the wall throughout all of our classes to remind students about the global ramifications of what we are doing by learning together in a classroom.
If your students aren't able to handle using Twitter like this, then you don't have a problem with Twitter; you have a problem with classroom management.
Will you get some of what was presented above in your feeds? Sure. But does that outweigh the benefit? No way; at least not from my own experience actually using Twitter everyday in class.
Social Media is not a monolith.
It doesn't tell you what to do. There are no rules.
There are countless ways to use Twitter; the key is that the use of the social media needs to be fully integrated into the teaching -- specifically for the purpose of teaching the content and skills of the course. So, the trick is to figure out how to do that for your course. And there's a good chance that a PLN model isn't gonna work for your kids.
As for WHY it's important for kids to take part in authentic social media (rather than the walled garden variety), I wrote a post yesterday where I tried to sort of lay out the framework of what we're looking at on the future grid of 21st century education. It's my attempt to argue what authenticity in education means now and for the future, and I'd welcome any criticism of it.
Anyway, Social Media is what you make it. It's yours to tinker with. So, use your imagination.
As for worries about how kids might abuse it? I really wouldn't worry about that so much, so long as you are incorporating ongoing discussions of digital citizenship into your lessons. After all, kids can write all kinds of nonsense on a sheet of paper and spread it around school, as well; they've been doing that for generations. Yet, I don't see too many teachers wondering whether we should allow them to write.
We're the teachers. We're the ones who need to model good citizenship in that classroom. We're the ones who have to model the effective use of Twitter and social media in our classrooms. It comes down to us, not to the technology.