I keep a lot of windows open on my computer most of the time. At work, I'd typically have email, web browser (with up to 10 tabs), several file browser windows, two simple note-taking programs, a project timer, one personal wiki, one plain text file editor, a development environment and, quite often, a database management window. That doesn't count Dropbox, TeamViewer and Belvedere, the apps that run in my system tray. Oh, and I forgot Wunderlist, my action list program.
Is that excessive? Perhaps. Maybe it's a case of laziness or impatience. If I close things down when I'm not focused on them, I need to spend extra time doing that when I switch away, and I need to spend extra time waiting for it to come back when I need it again. This is about speed and convenience. It's how I need my computer to work for me. Quickly and without getting in the way.
Mokalus of Borg
PS - I try to write my software that way, too.
PPS - It's why my note-taking program doesn't need a "new note" button. You jus start typing.