I know it is bannable, but I am not really sure why. I think it is just for security reasons, but I made an account for a friend who wanted to play online with me once because he couldn't make one, but he doesn't really play toribash. He never uses the account anymore, so I just use it as an alt when I am bored. I don't keep any items on it, because although he is a good friend, I don't want to risk it.
If you really trust who you are sharing your account with, like if they are a close friend or family member, then I would say, go for it. But just be careful, because you CAN get banned for it.
BTW, if any staff members are watching this, my alt that I shared is called cheeze7, I don't really care if you ban it or not, since I don't really use it anyways.