Pretty sure it answered your question. Why is the Mix boring? Well its because Storm is putting out a functional ball as opposed to an Angry Birds ball. As stupid as it sounds certain colors and or designs will also effect the way a ball rolls. Storm wants to put out something that's going to be an extremely entry level ball with a small block so you can 1. shoot spares with it. and 2. Have a legit option on the extreme extreme burn. Not only that but as noted by other posters urethane is going to be a hell of a lot more durable than a plastic option. Plus... Its not really Storm's market or anyone's market any more. Most companies are putting out urethane for their spare balls for that durability and performance level and going function over fashion.

I get what you're saying though. I had the original Ebonite Skull for the longest time and love the hell out of that thing and would but stoked if RotoGrip brought back the clear Grenade but at the same time.. I've also blown up 2-3 plastic balls in my time.

Side note... I'd roll the fuck out of an Angry Birds ball.