Psychologically,.... (okay,... just talkin' through my hat here)...
If your helmet is locked,... a thief assumes you are away for some time and at some distance...
I leave my helmet hangin' on the bars, gloves laying on the headlight, and even packages sitting out in open... Thieves will assume I'm close by, and bigger than crap...
I occasionally even forget and leave my key in the ignition...
...
'Course,... there is the possibility that nobody wants my stuff...
...