Well, yes, if you are there for your own sake, live-tweeting the goings-on is taking away from the experience. But if you are there for your followers' sakes--that is, if you are a journalist--then live-tweeting is the whole point of attending: you are serving as a proxy, telling them what's going on so they can experience the session "live", vicariously, through you. Journalists have never really paid much attention to speeches; before Twitter, though, it was because they were transcribing everything down on paper to analyze it later. Twitter just puts that transcription on the internet.
I think that's just as harmful. The 24-second news cycle creates an unending stream of fake controversies, corrections, retractions, and flamewars stemming from the rush to publish. As a reader, I'd prefer if journalists took notes, thought about the entire presentation, perhaps asked for clarification when necessary, and then published.
In the modern era, at least, it's not strictly true that journalists were busy transcribing speeches; it's standard practice for a person giving a speech (especially in politics) to give copies of the speech text to the press, usually before the speech itself. Typically it's provided under an embargo (http://en.wikipedia.org/wiki/Embargo_%28journalism%29), so the reporters can't actually report on it until the speech is over, but the extra lead time lets them start working on their stories early.
That's how you can see talking heads on CNN saying "we're told the President will talk a lot about taxes tonight," say, in the run-up to the State of the Union address -- they've gotten a copy of the speech from the White House, under an embargo. So they can't print the text of the speech right then, but they can allude generally to its contents.
You seem like you know what you're talking about. Why do you think the average live-blogger uses something like Twitter over a video app like Color? It'd take just as long to sit through a live-blogged session as it would to just watch an audience video of the same event.
I'm not sure your question makes sense. People following an event over Twitter can easily multi-task; they can just scan every few minutes or less frequently as desired. Following the same event with video requires lots more bandwidth (more awkward when mobile), attention, noise (or headphones), screen real estate, etc. Unless you're extremely interested in the event, watching live video is usually a big waste of time.
