Is science fiction useful

That depends on what you think is useful of course, but I find it useful... lots of Science Fiction has real science in it, so there is something to learn. Plus, all fiction teaches you about the real world as well, just in a less threatening way, usually by blowing it up bigger than life so it is easier to see different aspects. :) You can learn about real emotions and relationships by observing events in a fictional setting, because the author is real and the reader is real, and the fiction will connect to us... even if it is about aliens or time travel. :)

I find it also useful for relaxation. Reading in general is useful in teaching us vocabulary, and how to recognize and use our own language. Readers tend to do better at spelling than non-readers. I think there are a lot of useful things in Science Fiction.