Would you pick up a nade in real life and throw it back if you saw one drop at your feet? no you'll be too busy getting the hell out of there and how are you going to see the nade on where it dropped? they'll be a flashing light on the nade like on BF3 so you can know a nade is visible? or an indicator like MW3?
Of course there is a computer that can run arma 2 on the highest settings, the fact that there is a settings for hightest there is a computer that can do that I mean how was arma 2 created in he first place.