Here's a gif of me copying GFX data to code:

Here's a gif of me copying SFX data to code:

And here's a gif of me Copying music data to code:

So far, so good. Every bit of data copied also edits the clipboard, allowing these to be copy-pasted between instances of PICO-8.

But not so for map data! So here's a gif of me attempting to copy map data...

No edit to the clipboard was made. (Prior to each of these I had copied the -- TEST STRING TO COPY line to make it clearer.) Admittedly, you're not likely to ever need to copy map data between carts, but if you do (such as when working on a multicast game, or if you're transferring ideas from a prototype cart to the cart you plan to use for the full game) then it's gonna be real tricky to do so.

This issue was brought to my attention by the (as of now) latest episode from @Krystman on his advanced Shmup tutorial. Look around the 5:30 mark; he can easily copy the sprite sheet, but map data requires him to edit the P8 files directly, which works... but if you want to make the copied map data appear somewhere else rather than exactly where it was or only copy a small portion of the map, then that still presents a problem.

I too hope that this feature will be implemented.
The animation tool I created stores data in the MAP area, so importing and exporting data is a very cumbersome method.

(There is no way to reload() or cstore() a new cart in executable format.)