Take a basic arm curl. You train your biceps to tense, breaking the fibre of your muscle so that it grows back bigger, stronger, larger. Your biceps will be sore after doing this excersize, and, if done with any type of consistency, your body will form a habit. The problem with this habit is that when playing the piano, you cannot tense your biceps, else the tension will travel. Can you play fast octaves with tense biceps? No. Can you play chopin etude Op.25 No.6 with tense biceps/forearms? No.
You might be able to get away with playing at a basic level, or playing at an advanced level with discomfort, but honestly don't tell me that this doesn't hurt one's piano performing abilities. One needs not weight to play at the piano, but speed of attack with the precision of the 10mm depth of the key. Have any of you noticed the way your forearms are rock solid while playing the piano? That might tell you something.