The phrase 'writing as performance art' was coined by Howard Rheingold, who now enjoys an enviable position as a maven in discourse about virtual community and online life in general.
Rather than try to improvise, I thought i'd just quote some interesting stuff:
You can be extreme and say that being able to keep people entertained in a conversation is not the same intellectual skill that Shakespeare or Tom Wolfe had, that it's more like being a borscht-belt comedian. It's an extension of what television brought us, which is discourse as a form of entertainment.
That's a radical critique, and it's not totally inaccurate. However, among people who are perfectly good thinkers and writers, there is a subset who are not afraid to engage their readers and critics directly and entertainingly, in a more extemporaneous form, like a computer conference or a computer chat. It's a different craft. To craft a book or an article means going over those words again and again, very carefully considering each one. It's like creating a sculpture and making sure that every part is perfect.
You can't do that with conversation. You have to think on your feet, and once you've said the words, once you've typed them in, they're gone. They're out there. You don't have the separation between you and the audience that traditional authors have. Every listener and reader can call you on your mistakes and challenge you on your assumptions.
Here's the whole article if you're interested.