You should read about filament cold shock. It's actually just as common for a filament to be broken, say in an accident or bouncing down a trail, when the headlight is turned off and cold (thus brittle) as when powered and experiencing hot shocks. When the bulb is on and hot the filament is more ductile, which does create problems as the filament can flex more, short out or distort its shape. But they are also more tolerant of relatively small shocks while hot. It will be an interesting observation to test if being off does increase mechanical life. That should increase filament electrical life but I doubt a dirt bike would routinely have enough hours to burn out filaments just from being on, so it's mechanical shock that ruins them.
'91 Pickup - Imelda
'08 Tacoma TRD - Donna
'09 Kawasaki KLR650
'12 Gunnar Rockhound 29
"Our task must be to free ourselves from this prison by widening our circle of compassion to embrace all living creatures and the whole of nature in its beauty. Nobody is able to achieve this completely, but the striving for such achievement is in itself a part of the liberation and a foundation for inner security. " --Albert Einstein