Plus, how many shell can one individual open in a day ? I'm doing that once per day on a good day, maybe twenty if I have a lot of unplanned work on subprojects that needs to be done concurrently with my main task.
Yesterday just shy over 50, according to entries from `login` in the system log.
I do launch multiple interpreters just to get a fancy coloful cowsay on each launch. Which involves the fortune program, lolcat (via ruby) and cowsay itself (via Perl). I probably should optimise that into a single C binary for better startup times! :)
I don't use oh my zsh, but on one laptop zsh took 600ms to start. I narrowed it down to a strange "bug": adding even a single empty file to the custom fpath caused the slowdown. It bugged me so bad that I decided to try fish again, but this time for real. And this time it stuck with me. I like its architecture (defining functions to change its look-and-feel - great idea!) and of course its 32 ms startup time.
A reason is that I use i3 workspaces, with each workspace being for different tasks, so I don't want to reuse a terminal that has another context.
One issue with keeping a single shell is that the history is full of irrelevant stuff. If you run commands that take a while and print a lot of output, you want it to be as clean as possible. If I want the beginning of the command, I can scroll to the top, and I don't end up with some unrelated output.
I also take quite notes in vim everywhere, often next to long-running commands, using tmux.