Anything is better than nothing... I suppose - The large bore cans can be effective, except for these concerns:
1.) There is no way to be sure that the can is properly charged. This actually happened to me - I was throwing away an old can, as I was upgrading to a better system... I didn't want to leave it charged, feeling someone might be hurt by it someday, so I took it out back to "shoot" it off. The piddling little stream that evacuated the can was pitiful.
Had there been a bear attack, while I was relying upon that can: A better defense would have been to pee on the bear, as at least I would have had a more vigorous stream, and considerably more distance.
Other mechanical failure must be considered as well. A friend of mine, in the midst of an actual event, had the stupid little plastic trigger fail - It simply bent back below it's fulcrum point. Isn't that fun?
Relying upon a single weapon that basically has a single "shot," that is in itself *not* lethal, seems a silly proposition to me.
2.) Wind direction and velocity. ESPECIALLY with the large bore cans, with their wide blast pattern, you may wind up dosing yourself as much as, or worse than, the bear. This may well render you spicy, as well as defenseless, in close proximity to a bear that you have just pissed off even more than he was.
Now, to someone schooled in the thing, and able to keep his head in a violent emergency, this will be considered - But a panic stricken day hiker? Not so much.
3.) Every time I have encountered a bear, it has been a complete surprise - relying upon a single weapon (that doesn't even kill, btw), which one may or may not be able to get to, not to mention opening it's holster, drawing, and being able to deploy without injury to one's self, is not alot better than no defense at all. Most folks hang that can on their right hip, or around back... What happens if the attack is from the right, or if one is knocked onto one's back, with no time to draw?
Effective means in bear country require multiple strategic lines of defense - And one of those means must be in your hand. A .45/70 lever action rifle in a mountain sling is my primary defense (already in hand and ready to go), along with my early warning system: A very big, very bad-ass Wolf-Malamute (now passed on, a new one on the way). Equally important is a large bore sidearm (.44mag or better), a hawk, and a bowie, distributed variously around my body, just in case something goes sideways.
So "Better than nothing" is rather relative to truth, rather than comfort - A real "better than nothing" scenario, one with a chance of working against a really pissed off bear, would be a .30/30 and a .357mag, not a can of bear spray. And even at that, You may not come out of it whole.
Virtually every issue you raised about the bear spray you can raise against a firearm as well. Mechanical failure, ammo failing, being unloaded, bad aim, bad sights, being taken by surprise. Wind resistance and velocity wouldn’t apply in close conditions.
I’m not talking about is it the best thing to carry, or what you or my ideas about what we’d personally carry would be. I am saying for this guy, at this time of the year, to not at least have SOMETHING, even pepper spray if he’s a treehugger and anti-gun ecofreak, would have been better than nothing.