The J is a pretty valid piece. You get the second most score by triple clears with them. It's those pain in the ass square blocks that can fuck right off.
I don't know why, but I always imagine knights moving forward and then left or right, I always get infuriated for some reason when I see their movement described the other way around.
My beloved Ls and Js have been plenty useful in building up for a double t spin or speed comboing a ton of lines really fast to overwhelm someone. Give L and J a little love!