-Is there any real damage to forum?
Of course not. It very different from text plagiarism. With plagiarism of posts, forum become more polluted with spam and repetitive quotes. There become harder to find anything useful and even to stay on forum (it's cringe to see one sentences many times).
Plagiarism is against rules because the person who plagiarize benefits from what copied and pasted without any effort. Plagiarism is against rules because it's kind of theft, not because of spamming. There are many spammers in the forum that are not banned. But high quality posters are banned even if they plagiarize only once.
Stealing signature design is kind of plagiarism and kind of theft.