I don't think I've seen many (any?) people say they bring bear canisters for day hikes up Whitney (I'm going early Oct, if it matters).
I suppose it could be useful for marmots, if I put the pack down.
What I read online suggests they're required. Any comments? the ones I've seen in stores seem fairly large, and awkward to open frequently especially if you're in winter gear, but I guess if they're required, they're required.