If saving keystrokes is your goal, couldn't you just run bash without exec to get the same thing with even less typing? The only difference is you'll have to exit bash twice when you're done.
Non-FHS systems like Nix require a lot of per-executable environment setup. This basically means that every executable gets wrapped in some kind of shell script to perform that setup, and some things can end up wrapped multiple times. It's critical that those wrappers be able to `exec` from one to the next or your system would quickly be awash in bash processes just idling waiting for other things to exit.