Well, sure, robots.txt is not the best solution, but it works and helps a lot when you got msnbot or yandexbot that takes more than half of the requests of your mediawiki (differences between revisions), your gitweb (commitdiffs) or your phpBB installation and kills the performance...

Bored of having our machine killed by those bots, we use some robots.txt.

Sure, there are other solutions (proper blocking), but this one works perfectly fine and avoids having to modify 3rd party applications that we are running for an open-source development team.