The advantage to hitting from the opposite side of the hand of the pitcher is you can't really get busted in with a fastball, the ball has to come across the body and in to your zone, so you SHOULD be able to tee off before it jams you.
As for why lefties have such a tough time with other lefties, it's simple. They just don't have much history facing off against left handed pitchers. All their life they grew up facing right handed pitchers. Then, when they get to college/pro, they see the mystical left handed pitcher and are helpless at the plate.
Some adjust, some don't. You want left handed hitters, because though left handed pitching may own them in head to head match-ups, the majority of pitchers seen are going to be right handed, so they'll likely have the platoon advantage.