My grandfather never would have armed himself to protect himself from these criminals. He probably wouldn't have needed to.
I don't know about that. I suppose it depends on how old you are: my great-grandfather likely had to deal with bootleggers (he was a cop during/prior to WWII), and I know my great-uncle did (he was a Park Ranger back when they regularly had to deal with violent poachers and bootleggers - 20s and 30s).
I'd personally be more worried about meth cook sites than pot. Meth takes less time and less area, so it'd be easier to "stumble upon" when there's an armed guard or booby trap. With regard to outdoor safety, at least a lot of the small-time stuff is being done with the "2 liter bottle" approach than w/ the more environmentally dangerous cook site.
Depending on where in the country we're talking about, pot is a different story. I'd not generally be concerned in (say) WA or CA, because everyone there smokes it and/or grows it. I'd avoid it if I found it, but I'd not be all too concerned w/ being shot. Still, going armed is the best policy.
I've stumbled upon a pot grow before, and I've got a friend who stumbled upon (on a back mountain bike trail) a mushroom grow of some sort (he just narrowly avoided running into a tree felled across the trail which had the branches cut off like punjee sticks). I suspect he'd have found a lot more than mushrooms had he bothered looking.