It would make sense if dragging a .p8/.p8.png onto pico-8 ("pico8 file_path") would cause it to cd to the containing directory, load the cart, and run it. In particular, this would allow to associate pico-8 with the .p8 format and/or start it from external applications (e.g. to use Notepad++/Sublime Text for editing code and have a key to launch pico-8 with the current file loaded).

Some kind of auto-reload (reloading the current cartridge if it is changed on re-focusing pico8) would be better for editing in external applications to avoid sitting trough the booting sequence every time :)
Auto reload is not very retro-consolish though... :)
I don't even mention live editing... :) (Zerobrane Studio can do that with several lua based engines - I don't know if something can be hacked together with pico8 too)

Another possible solution could be just skipping the "boot" altogether if pico8 launched with command line arguments. (I suppose the boot sequence is just for the show.)
Or (to keep the retroish feeling) only skipping the boot sequence if there is a specific argument like "pico8 filepath -devmode"