How many computers do you use? Having to switch between zsh and bash mentally when you from your dev box to a server can be jarring. For some people it's worth it, for others it isn't.

just stick an exec zsh at the end of your bashrc, wrapper in an if statement if it's not available on all machines on your environment.

do people have the same attitude to screen/tmux or vim/emacs (vs nano)?

I don't think untog was talking about the overhead of executing zsh upon starting a new session so much as the mental overhead of the mode change from zsh specific features to those of another shell.