Funny, hardcore SL people ask that of ourselves. For many of us this is the time we don't watch television anymore. SL's a damn better use of time than watching television, its sociable, interactive, and allows creative individuals to express themselves. Some even make a living at it and use it as an extension of their RL enterprises. There are fashion designers that test out their designs in SL before RL. There are professors who use it to teach math. There are people who use it as a simulator for real-world emergency exercises as an alternative to more expensive virtual world systems (e.g. MIT's CAVE system).