The reason the emoticon became popular is that with the advent of the Web, people began writing far more than previously, but not better.
So the emoticon replaced any gift for nuance lacking in their writing, which was always missing, but did not get nearly as much exposure.
Everyone is a writer now. For better or worse.
Those of us who think of writing as an art and therefore disdain "smileys" should remind ourselves that more people are writing more often than ever in the history of our species. It's no surprise they adopted the emoticon pretty quickly to fill in the gaps in their skillset, where writing and the art of letters is concerned.
Also, for what it's worth, most of us write differently in different arenas. I wouldn't the same way in an email, on Twitter, on IM, in a short story, in a music review, in presentation for work.
- In response to "Death to Smiley" by Mary Elizabeth Williams