I've seen a guy literally chase a bear at the Portal, chase. This was in broad daylight and not during the wee hours of the night.
I've carried a large can of bear mace before, but only had to use it once (in Yosemite no less, sorry rangers, didn't know), and against an angry buck, not a bear.
When I first went up Whitney, I asked a ranger at the Visitor's Center whether the mace was necessary. He said he doubts it, but also said that I should carry it if it'll give me peace of mind. I think that's good advice. I rarely carry it anymore, but if it'll let you sleep at night, bring it along. I worry more about the other critters out there like mad deers and mountain lions.
