I use them very often in Emacs instead of keyboard macros. They are useful whenever you have to perform the exact same operations in several places simultaneously visible on the screen[0]. By operations I mean both simple ones, like writing something, and more complex ones, like navigating/deleting whole words (of different length), etc.
Mostly I use it for quick renaming of variables, string fixes, and some structural changes in code.
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[0] - multiple cursors, at least in Emacs, can work off-screen too, but I don't like not seeing what's going on when I do changes.
I also do this, though I use iedit-mode rather than multiple-cursor-mode.
However, I do like to point out that it's really just eye-candy for stuff I was already able to do in emacs. Previously, I would have used either keyboard macros or query-replace-regexp to do the same things.
Mostly I use it for quick renaming of variables, string fixes, and some structural changes in code.
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[0] - multiple cursors, at least in Emacs, can work off-screen too, but I don't like not seeing what's going on when I do changes.