I do. I've always felt that I'm doing something wrong when I do this, but I much prefer not having to switch files all the time and navigate folders etc. looking for things. With everything in one file, I can just use regular within-file search (and M-x occur in emacs which is a bit like clickable (and editable) grep). If I need "focus", I just use narrowing in Emacs (shows only the highlighted part of the buffer, with handy shortcuts for narrowing to current function/class). If I need to see something side-by-side, just split or clone the buffer. I loathe working on those java etc. projects where you have a zillion classes each in their own file, many with maybe a handful of members each; whenever I switch files I feel like I'm losing context.