As one of the commenters on the OP mentioned, live-tweeting is often used as a means of note-taking, and much like the commenter in question, I find it's crucial if I want to stay engaged with both the presenter and the content. I could keep it to myself, but if it's contributing value for others by making it public (and it usually is), why not? I do have total empathy for the agitation it can cause a presenter, though, so I do my best to model attentive behavior anytime I'm not jotting down a note.