The E92 M3 we had had the aeros as standard.
Scraping or streaking can be fixed pretty easy. Get some vinegar and give the blades a rub and the same with the windscreen. Amazing how much crap comes off them, yet you cant see. Then put some rainex on the windscreen and it will be smooth as silk. As for when to change, when the blades start to perish and they are no longer effective.
Grease is the main culprit from trees road contaminants etc. Vinegar will eat through it and sort it although it does smell. You can get products that do the same thing but you will probably have vinegar laying around and wont have to pay the £10-15 difference.