I don't have the time to explain why strikes from the bottom isn't as effective as strikes from in someones guard... or half guard. I didn't think something like that needed to be explained to people who have so many posts.
Pretty simple really -- there's no power/weight behind them when you have somebody's bodyweight ontop of you -- pushing you into the mat. They are more of arm punches than anything of significance.
They can cut/bruise, just like any other strike though... you're simply tearing the skin when the glove rides along the face. No damage behind them though.