The only problem with these questions is that different markers have different 'scales' - some are tight and some are lenient. However, they base their marking on a more or less identical set of criteria - whether there was sufficient elaboration, evidence and proof, whether the argument was presented coherently and logically, etc. Some tests (like geography exams in Hong Kong) present a list of points that may be raised by each side of the argument, and students are awarded a point for each point that can be found in the marking scheme. This makes it completely fair since the marking is completely standardised,
I do in fact agree that in things like university exams, answers based on opinions are impossible to mark and could be marked unfairly. Although in some tests opinion's are needed. Like for instance a psychology test. They make a decision on whether the person is sane or not by looking at there opinions. For instance the question might be what their favourite colour was. There is no correct answer, but certain colour choices might indicate an angry personality. This is all i have to say at the moment, but opinions are needed in some situations.
Opinionated questions are truly unfair in every way you look at them. While you may get to express your personal opinion, the reader may disagree on every view. For example, if you are writing a paper for your English teacher/professor they have the power to lower your grade just for stating what you believe in. So technically this would be in violation with the 1st amendment, which is just one of the several amendments schools appear to violate.
Opinionated questions anr not fair for trillions of reasons. Firstly they do not have a right or wrong answer. Secondly they can not be graded. Thirdly since they can not be graded they are pointless. Fourthly they may cause problems with kid on higher levels like me who always want to have the right answer. But since opinionated questions don't have right or wrong answer you can not get them right. Even thought YOU might think that they are an easy question to get right. But you can't get them right or wrong no matter how hard you try because they are only your personal opinion EXCEPT when a child wants to be popular or "cool". When that happens then is the child writes what the "coolest" response is therefore not making it an opinion.
From the mind of an eleven year old genius.
P.S. If you watch child genius you can tell they are dumb in certain sections. So I think that opinion Questions are right up their alley.