@Solonit, because fully expending SP is the same as overexerting, It's not just getting tired, that happens way before your SP ends. Also eating food gives you energy beyond the limits so it's almost impossible to run out of SP unless you really push it.
In the novels Kumoko had a skill that allowed her save a nearly infinite amount of SP by eating, so on most fights at her level, she'd barely even touch her reserves. SHe had nearly infinite stamina, and would only really expend it on extremely hard fights, that's when her reserves dwindled and her SP would be consumed.