You can get a skull fracture just moving your bike in a parking lot.
The only time i go without a helmet is when I am literally going around the block to just give the bike some turnover - seems it sits there for weeks without being used.
Otherwise always a full helmet for noise reduction ( serious issue right there ) and bugs. They sting and one embedded itself in my neck - had to extract it like a tick - that was the last straw. Florida has insect aviators all year round.