Personally while there are some task that's easier with multiple cursors, I have personally yet to find any use case where regexp replace in selections doesn't work as well.
Multi-cursor is more flexible than rectangular selection. You can skip or delete words of varying length etc. It is certainly less powerful than vim macro and regex, but it's easy to use because
1) there are less shortcuts to remember
2) no need to think too much like figuring out regex for simple tasks
3) it gives instant feedback
With regex, you need to plan everything you need to do and then surgically do it.
With multiple cursors, you only need to know (roughly) where you need to change, the rest you figure it out on the fly.