When you’re as small as a pill bug, it seems like rolling up into a little ball might not be the most practical solution to every problem. I guess you’ve got to try something when you’re faced with an apocalyptic crisis! Still, it seems to have worked pretty well for the old Armadillidiidae family, who have managed the trick of being terrestrial crustaceans for millions of years. Yep, these guys aren’t bugs, insects, or millipedes at all– they’re more closely related to crabs & lobsters (although they aren’t decapods either). So, rolling up into a ball like that might not be so much a practical defense against predators (it seems like “large” creatures like birds, rodents, and lizards would just eat them either way), but is perhaps a means of efficient moisture conservation during dry times. But I’ll bet being a little armored ball is probably still an effective deterrent against smaller predators and pests, like centipedes, ants, or roving spiders.
Incidentally, these little guys are entirely harmless to humans, so don’t fret if you encounter them while gardening, or find them hanging out in the basement. They can sometimes be a bit of an agricultural nuisance since they like to eat vegetation, but they tend to do a lot of miscellaneous rotifer-kind of eating too. They won’t bite, and they’re not poisonous!
While most terrestrial isopods are usually pretty small, there are giant marine isopods out there in the world, mostly in the deep ocean. Their carapace gives them a little bit of a malevolent countenance, but they’re usually pretty benign too, and hang out on the ocean floor having a good old time. I did come across another type of small marine isopod in my research as well, but I think I might keep that one under my hat for the moment, since it might make for an interesting comic in the future. Mysterious!