I would imagine that windshield glass has to meet certain safety aspects and hardness requirements. I know back in 2000 when I got my PT Cruiser many of the owners suffered chips and cracks from stones hitting just below the windshield wipers on the passenger side of the car, mine being one of them.
If they are small pits that you can feel with your fingernail, you're not going to be able to polish them out. Eastwood makes a polish that will remove streaks made by old wipers but you have to be careful not to polish too long/hard in one spot or you can actually damage the optical clarity of the glass. It is like jewelers rouge and uses a special buffing wheel which they sell too.
http://search.eastwood.com/search?w=...olishing%20kit