I do think in some cases it's a defense thing. Most livestock guardian breeds that I've met do have some "give" to their lips, though they don't droop like that of the mastiff. My border collies, in comparison, have really tight, firm lips and if they were to be grabbed by another animal there, they'd be stuck. Heck, even dogs like a Doberman would be hard pressed to defend themselves against another animal considering their tight skin.