this is a normal boost of mentality. coz u know someone will help u if u fail.
but again, it depends on individual.
i have a fren that doesnt want to be spot coz he knows he will tend to fail more.

there are another common call for spotters.
dumbell bench presses and shoulder presses.
grabbing them buy the elbows or triceps area.
i remember spotting an idiot who tries 40kg per side incline press.
i think i train my bicep more than he train his chest.
it was just 4-5 reps. showoff idiot.

haha true, but he is a showoff iranian. =.=