First off the OP wrote about illegal Mexican bears and as far as I know - there have not been any grizzly sightings in Mexico in recorded history
I consider bear spray to be far more (dangerous) to carry around than the (danger) of meeting a black bear. I have had conversations with bears in the Sierras and I consider myself lucky to see a bear and converse with it. I've heard of people getting bear spray on their body or in their eyes and it sounds pretty horrible. A bear should never be close enough to you to spray - instead you go ballistic when the bear is still 20-30 feet away and throw rocks at it AND throw a large rock down onto another rock - that is a rockslide sound and in my experience - all bears run from it.
THE ONLY PROBLEM BEARS THAT I HAVE EVER ENCOUNTERED WERE IN NATIONAL PARKS.... and thats because of the way rangers and visitors treat them. Most animals avoid the parks, it seems, and hang out further from man - but not bears. I've never heard of a mountainlion incident in Yosemite, for instance, but they are thick other places in the Sierras. Somehow its the "teddy bears" that we have our problems with.
Unless you live in grizzly country - forget about bears. I mean don't be foolish - just don't worry and for (insert fav diets name - sake) do not shoot at bears, as they take offence at being wounded and if you're not bear hunting, whatever you're carrying probably won't drop a bear fast enough to keep it from killing you anyway. Sometimes I carry in bear or lion country when I have my dog or wife along - to protect them, but mostly to make a loud noise to scare things off - my concern being not to kill anything, but my .44 growls real loud. Anything that doesn't leave the area after a .44 goes off should be considered sick or insane and you maybe better shoot it - it could be rabid.
These are my own opinions based on wisdom earned through many wrong decisions. Your mileage may vary.