"People adapt language to suit their ends. One of those can be play, and that seems to be what's going ... temporary ones develop, and those changes don't seem harmful to language in general. How could exercise of flexibility be harmful?"
Regardless of language forms being harmful or not (and I agree with you in spirit), my example touches on another trend that I'm just as concerned with: Loss of personality or individuality.
In my opinion, the less you type (say, in an email, chat room, or forum) to someone else, the less of your personality comes through to that person. While smileys and abbreviations can be apt and sometimes expressive enough, I still think that if what you type is close to what you would say out loud, you express yourself better.
Internet abbreviations, shorthand and emoticons are so homogeneous that the writing of people who rely too much on them turns into a kind of inscrutable code that doesn't represent them very well as people.
At least, those are my thoughts. Dissent or discussion is welcome.
Jim