When I have a full backpack, and making my transition between classes, it is always a choice of holding my helmet in my arms, and wearing it atop my head.
The benefits of the former are that it is more comfortable to hold a helmet, that it keeps the hair one step closer to its initial state, and that one doesn't look like one needs it to navigate the halls of one's college.
The benefits of the latter are that it is a hands-free method of transportation, and that it makes one readily identifiable.

Somebody told me today that there was something wrong with my helmet; the hard plastic casing had come off some months ago, leaving just the dense styrofoam interior. The concern over my safety among those present was rather high. On my way home, my chain snapped within a block from the school. I had to push my bike up the hill, helmet on my head. For my own part, the choice is almost always to wear it. It identifies me as a cyclist, and more frequently brings up the cycling option in others' minds.