There are different types of communication. You certainly can learn interpersonal skills that way, but blogging is a great mental exercise on getting your thoughts on paper. The only way to become a better writer is to write, and blogging is just that. Tweeting is actually another great writing tool; it teaches you to communicate while being brief. And teaching is always good -- it helps you learn the subject better and exposes you to different perspectives.
Basically, if you want to ever be more than a code monkey, you need to speak up both in person and online.
"but blogging is a great mental exercise on getting your thoughts on paper. The only way to become a better writer is to write, and blogging is just that."
So is writing an email. In a typical day I probably send 5 important emails. Writing opportunities abound!
Not to even start on writing designs, memos, technical briefs, position papers, meeting notes, technical docs, user documentation, etc, etc.
The opportunities to practice your writing skills at work are limitless. I'm not opposed to blogging in any way; let's just not pretend blogging (or tweeting) is in anyway important or necessary for building communication skills.
Basically, if you want to ever be more than a code monkey, you need to speak up both in person and online.