I use a single 20" LCD and working in PAL the biggest practical preview window is at 50% scale to get a good window layout (I actually prefer single monitors, next on my list is the Sony 23" Widescreen!)
As I am working I scrub the timeline and picking up on big changes is OK but sometimes the subtle changes make you squint (looking at the preview window), being able to glance between the preview and external would work well for me. Setting ext preview on and off all the time just becomes a pain.
It's just good to have it there all the time and forget about it - make sense?
Paul
PS. It would help if the preview window had the integral scale option of Sound Forge for the preview window