> About a year back I moved from 120 back down to 100. The rationale was that, even on a large monitor, it was difficult to fit multiple source files side-by-side at a font size I could read comfortably. On a laptop monitor, it was hopeless.

It very much depends on the screen. With a 32 inch screen you can fit three 120-column views at the equivalent of 100 DPI at default zoom. Or even better, six 120x40 views.