I rarely find myself wishing to split my screen. It's not that I don't know that I can, and occasionally I will (especially if two pieces of code happen to be VERY tightly coupled, with lots of passing back and forth between the two). In general, however, I find that since I am only reading one line of code at a time, having two editor windows up is pointless. It's like having two browser windows instead of two tabs in a browser window.
Plus, with most editors that can jump in / out of functions, it's not like I'm losing my place. In fact, my editors all save my place in files on exit.
Unfortunately, at my current job, we don't seem to believe much in unit testing. There's some indication this may change in the near future.
Additionally, I have two screens, and since I mainly use a tiling window manager, 80 columns works... okay. But 80 columns isn't usually where my splits end.
Interesting point with the same file in columns. It's rather that I do that. More often I'll be looking at the same file across two branches. I am kind of surprised that we don't see more plugins doing 80 column lines, in sequence, to fill the screen however. More lines on the screen seems to be the craving that vertical monitors + high-res monitors aim to satiate.
80 columns of text is wonderful in my dual window vim with NERDTree on the left. All my screens are some version of the 1680x1050 aspect ratio and it fits perfectly. I'm always using 2 windows for simultaneous code/test writing, diffing, and multiple windows into different parts of the same buffer, multiple files open etc.
Plus, with most editors that can jump in / out of functions, it's not like I'm losing my place. In fact, my editors all save my place in files on exit.