The idea of General Studies is to examine stuff not taught in other subjects, and indeed that which is hardly teachable at all. To say that it's a waste of time because it's not like other subjects is rather narrow-minded, as it's not meant to be like other subjects! I would say that it is a mixture of an intelligence test, a general knowledge test, and a general skills test (i.e. essay writing skills, comprehension skills). Surely it is good to have a qualification that proves you are good on those!
I don't see how it is much of a waste of time as you need/can do hardly any preparation for the exam (just glancing over a few past papers to get an idea of what it's like was all I did), so even if it is useless it's really just the three hours it takes to do the exam that you "waste"!