Reading is NEVER a waste of time.
Even fiction or children's books. Reading expands your vocabulary and helps grammar, as well as gives you other ways to express yourself using language.
Plus, it's a great way to spend time that isn't connected to other people. Most things we do in today's society, even sitting alone in a room on a computer or texting on your phone--you're usually still communicating with other people, and sometimes it's nice to just take a break from that. Similar to what Pweep said but 'happiness isn't a waste of time. Isn't it the point of life?'