This is indeed Incredibly useful!
I have a suggestion for the way the "remove cursor" command works. It would be nice when a cursor/selection is removed the next cursor/selection that gets activated is the one that was closest to the one that was just removed.
For example, imagine I have 3 cursors positioned in a large buffer where the first one is on the first line, the second one is on the second-to-last line, and the third one is on the last line. If I have the second cursor activated and then removed then what currently happens after that is the first cursor will become active. However, activating the third cursor instead would make more sense to me.
That being said it's a minor issue and overall this package feels very natural to use. Good job!