As a former humanities student who did study, among many other things, studies in religion including some stuff about mystery cults and secret societies, it depends what you mean by useful. If your degree is only in really esoteric stuff then it doesn't make you very employable, but becoming employable isn't the only reason to go to uni anyway. I would have liked to do a grad dip in studies in religion if I could because I found it fascinating. My goal would then have been enjoyment and personal fulfilment. Humanities in general has more value than that though. Studying and recording human culture is important. It teaches us about the past and who we are as people. If they stopped teaching these things we would lose a lot.
Of course it's useful! Quite apart from learning more about humanity and how our minds work, it teaches research skills, analytical thinking, the ability to digest and debate information.... all good skills to have I think.
And at the basis of all science and really the uniting feature of all great scientists, is WONDER and the ability to question. Studying myths and magic is necessary for science.
I think it is interesting and useful to spark the imagination by studying myth, magic, and mystery. Who knows - one day all this "unscientific" stuff may be backed by scientific proof, but we'll never know if we stop looking into it!
Waste of time & possibly, dangerous getting into subject matter about which we know nothing of its' ability or intent.
Just imagine a job interview: 'I see here by your CV, Mr Potter, you've a Masters Degree in Hocus-Pocus & Tomfoolery'? I'm very sorry, but I feel that's not suitable enough for your application as Chief Stable Sweeper-Outer. Good day to you!