My monitor's resolution is such that two 80 col windows leave 37 columns spare (and that's including the window furniture). So an 80 column terminal and a 100 column emacs fit very nicely together.

And a lot of the stuff I'm working with is log files with much longer lines than that, so I'm out of the habit of assuming things are narrow, unless I'm working in a terminal. I should try and bear other people in mind a bit more though.