I wouldn't mind it doing 1-3 points of damage. Something that says, 'Hey, you just got hit by a wall.' But not enough that people will be tempted to do it on purpose to hurt their melee buddy because they think it's funny.
Well I gota disagree with you. Taking unrealistic and bare minimal damage from a giant pile of rocks is cheesy, don't you think? About as cheesy and standing under them and having them fall through you.
Maybe take enough damage to where you don't want to get hit by the rocks, but not enough to ruin the gameplay if someone does try to grief and is successful.