Well, no matter what reasons one may come up with, plagiarism is not permissible under any circumstances at all. It’s as bad as cheating in an exam, probably even worse than that. If you are actually looking for an answer to the question, “Can plagiarism be allowed under some circumstances?” you might not get even a single answer in affirmative.
We are taught from the beginning that we cannot touch or use our siblings’, parents’, friends’ and even strangers’ stuff without their permission and even if we do after taking permission, we should thank them for allowing us to use their stuff. Same is the case with content and other stuff shared publicly by others. If people have been nice enough to share their knowledge with us, can we not be courteous enough to at least give them some credit?
In the case of knowledge and content, the author has probably shared his/her work for the sake of spreading knowledge, but not many of them would allow their stuff to be used by others with their own names. If you make a painting worth $2000 and somebody else gets a copy of it and writes his own name on it, you would go to every extent to make sure he is penalized. So is the case with plagiarizing content. It’s never a good idea to use others’ stuff and that without mentioning anything about them to give them the credit.
Students might think that since they have a poor instructor and since he/she fails to explain concepts, t should be allowed for them to plagiarize content from other sources when working on their assignments. That’s not true. Under no circumstances can plagiarism be allowed in any field, be it a student, a professional or an artist. You cannot claim the stuff to be yours that is not your creation.