Thanks for bringing this topic up, as it encouraged me to try it, and now I've switched to using my central 22" monitor in portrait mode, while the ones on the left and the right are in landscape mode.
It turned out that there were several applications that worked better for me in portrait mode. The obvious one is writing (e.g. with WriteMonkey), as it allows me to see a lot more of the text--and therefore the context--of whatever paragragh I was working on. Another one is reading PDFs, as mentioned before, and most web pages (especially due to the latest craze to format web pages in tablet-sized screen chunks piled up vertically - you know what I mean...).
Also, any apps that have long lists (or long pages) work better in portrait mode, as there is less scrolling involved. E.g. WorkFlowy (to-do lists), ConnectedText (list of wiki documents, or long individual docs). And of course email.
I was surprised to see that even Google Calendar worked better in portrait view, as there was no need to scroll, so all the events of the week were visible at a glance.
But I'm sticking with landscape view for Dopus, especially with dual panes and file preview on.