I think it is aspirational and cathartic for straight-laced people to see slacker-dudes. We can live vicariously through them without getting into the jams or working the same dead-end jobs. I love Clerks, Mall rats, Drug Store Cowboy and just about everything made on slackerdom. At the end of the day though I want my life. Hollywood is good at cashing in on this type of escapism.
Edit- It reflects the true to life image of the some of the post-adolescents who haven't found their way yet. However in real life these people aren't all that likeable. I don't know any miscreants as entertaining as Bill and Ted. Most of these slackers out here in real life end up burnouts, or convicts if they don't straighten out at some point.