It is well known that people who advertise who is or is not in their
killfile harbour serious delusions of grandeur. Why post "plonks" in
the first place when you can just do it and move on? It's a "look at
me!" thing.

I agree with this.

It's also a common technique to avoid answering arguments that one
can't answer. The "plonk" is rarely done in this case (I am positive,
for example, that Crinklaw is reading this despite his "plonk" of me
... heh!), but stands as the admission of defeat, but done in a way
that soothes the ego (and alleviates the growing cognitive dissonance
problems).

I don't agree with this. I see it done all the time to avoid seeing
arguments from people who can't or won't recognize their arguments don't
hold water, much more than I see it done to avoid admitting defeat.
Both happen, but the former is much more common. At some point, people
realize that reading the posts of some others raises their blood
pressure. Killfiling, done silently, is a perfectly legitimate solution
for that (in my opinion).

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt