There are so many keyboard options for moving through source which you don't have with a mouse...

- Page up/down (and especially with home row shortcuts like in Emacs and Vi)

- Brace match jumps

- n line jumps up or down in Vim

- top/mid/bottom of view jumps

- page shifts up and down without moving the cursor

- top/bottom of file jumps

- jump directly to a line in a file

... and more

Seriously, put an Emacs or Vi power user next to a guy with an IDE and a mouse, and it's no contest.