I have seen the 'iconing a character I don't know' thing mostly in the CLAMP fandom. Maybe Yu Watase and Arina Tanemura art as well. Generally that distinction goes to any artist whose stuff is just considered extremely pretty. I've seen it happen a couple times in Rumic fandom (this one time about a year ago this girl posted an entire batch of 'Kikyou' icons and they were all Mana from "Mermaid Saga), but not too often.

I don't get it either! I've done it for icontests if they're really low on entries, or when I'm asking for requests or as part of like a themed set. However, no 'Gee this image is pretty. Even though I dislike the character, I'm going to use it.' I agree it's a waste.