Over the last 9 or 10 years I've definitely seen bears in Sierra wilderness areas. The comparison between Yosemite Valley is probably better with places like Tuolumne Meadows or Cedar Grove, where I've also seen them .. these are more car-camping areas with lots of, ahem, tourons
but also lots of bear boxes.
The last bear I saw was up at Saddlebag Lake last summer, at the trailhead. I'm sure nearby campgrounds draw them in. Good thing the place has bear boxes ..
That doesn't mean I always carry a bear canister -- it depends on where I'm going and what the regulations are. Sometimes if a canister is just 'recommended' and the ranger makes a good case I'll bring mine. But if I can get away with hanging food, then I will. The method I use was taught to me by a grizzled old dude in the Sierra Club, and is illustrated here (scroll down for the 'PCT Method') -
http://www.backpackinglight.com/cgi-bin ... nique.html
Re: Ursacks - I bought one of the yellow aramid models back in 2001 and think they are great for protection against marmots and such, but have heard of several failures with various models of theirs. I don't trust the company for anything but critter protection.
The lightest canisters out there are
Bearikades. Spendy, but the national parks also rent them. They have never been compromised by bears.
BareBoxer also makes a small canister, kind of a scaled-down Garcia but it's limited in capacity -- I like it for short solo trips.