I have an immediate, almost visceral disagreement with the suggestion that a mouse is more efficient, but I may be wrong. For sake of argument: I think the "just pointing vs. using the keyboard and getting more thought involved about structure disrupts the sense of time" argument may have something to it. Still, using a keyboard well counterbalances this by being able to take advantage of greater structure - I very frequently use elaborate keyboard macros, for example, and the mouse doesn't have any clear way to say, "Ok, the thing I just did, apply that in a batch to the rest of those that look like a C function prototype." The mouse is more immediate, but you can do more powerful manipulation with the keyboard if you know the program's keyboard shortcuts, etc. (That's not necessarily a good trade-off for many kinds of programs, but for a text editor / IDE, it's probably worthwhile.)

While there have been some attempts to do that with mice (such as mouse gestures), by design, mice are about position, changes of position, with a few buttons as flags. There aren't enough distinctive signals.

Also: The Emacs mouse support is not great. It probably hasn't been a high priority.