I think it depends on the person. It's not harmful by itself, but some people, like with food, alcohol and sex, use it to escape from their problems, and spend their days stoned enabling their procrastination and irresponsibility. The satirical song "Because I got high" by Afroman is a good example of this. So in the end, if you are a "functional pothead", and get stuff done, are happy with your life, it's fine, while if you have a mountain of bills unpaid, a house that is a mess, and day after day you keep spending all your time getting stoned, and complain how you don't get anything done etc, well, weed is bad for you (I'm using an acquaintance as an example :( ).