For fairness sake, here's a PM I got from a user by the name of "dzeiger":
Apologies for sending this via PM, but looks like I'm currently restricted from posting in that forum (probably because my subscription is currently lapsed, until either they get payment options worked out, or there's enough content to justify me paying for another movie or whatever).
Anyway, for my data point, I do not run my browser in fullscreen mode--I prefer to be able to access other desktop icons or windows without having to alt-tab or whatever. The window is still perhaps a bit wider than it is tall, but most artists doing widescreen images would end up forcing me to scroll or make the browser fullscreen (or both, I've seen more than a few widescreen shots that I have to scroll through even in fullscreen mode).
All things considered, I'd rather scroll down through a traditional, known portrait-mode format than having to be messing with browser sizes that I may or may not still have to scroll through.