Aren't phrases easier to type on mobile devices than letter / number combinations? I always find it annoying to have to switch to different keyboard modes (text / numbers), or having to hold the letter / number key for a few seconds.
The error rate on a touchscreen keyboard is high enough to really become a problem at 20+ characters when you only see the last typed character (no password review).
It could also be keyed in via something like Swype or SlideIT in almost as fast as it could be keyed in on a computer keyboard.