I just created this account to say that I agree wholeheartedly 100%. People always ask me, "How do I get better at Vim?" or say "Vim is too hard" etc, and I always say `set mouse=a` isn't just a gimmick, it's a _very_ useful feature...
Although, it would help to show beginners exactly why abandoning the mouse will be better in the long run. It wouldn't do well for them to always rely on it in place of more efficient commands. Some people might always use the arrow keys instead of hjkl, for example, when hjkl will be better in the long run since they are on the home row.
I don't believe I'd write usefully larger amounts of code by using hjkl instead of arrow keys, and personally I am always going to use the mouse when I want to move more than a couple of lines.
What changed my opinion was watching videos of people using vim on youtube that made me interested in learning it for real. It made me realize how much more efficient it could be.