Fair enough, but can you imagine the press hoopla had Apple introduced this feature?
Check out this video:
http://www.engadget.com/2011/10/19/ice-cream-sandwich-face-u...
It seems quicker than swiping an unlock code.
I think it could be useful if the camera has a zero shutter lag as it is in the nexus. A scenario coming to my mind is maybe hands-free interaction with voice commands while driving. Anyway, it don't think locking a phone has a security connotation here, but it is a way to avoid any accidental launch of application/typing.
Lock the phone and carefully look at the screen from a few different angles. Chances are that you can clearly make out the greasy trail of the unlock pattern. I switched back to a PIN code when I noticed this.
Pick up phone, hold it out at arm's length in front of you. Hold it real still. Now wait. Bloop bleep boop now it's unlocked!
Compare with:
Pick up phone, swipe your unlock gesture. Hurray!
For something the user is doing constantly, many times, every single... I just don't see why anyone would pick the more uselessly time-consuming (and more effort required) option.