Yes they are worth building, especially if you are winning a lot of games. Bleachers tend to sell lots when you are winning games, and sell badly when you are losing games. So depending on your situation in your league, decide to build bleachers or not. I personally think building the luxury boxes and courtside seats better are a better idea.
They are cheap because they are for fans that only want to watch you when you win, they aren't dedicated fans that are happy to buy luxury boxes etc.