Personally, I believe it's a matter of choice. These days, I only wear one on rare occasions.
I prefer the girls to live free. So much more comfortable that way. Granted, that's easier to do when one lives in a cold climate where sweaters, jackets, and other forms of outer wear are common, ...but come the summer time, if I'm to be stuck in air-conditioned environments, ...there are always circular band-aids to keep me from taking out an eyeball or two.
As long as I'm not required to run, bra-less will always be my preference