Moderation......... that's the key.
There is no question that pot has ruined some peoples lives. Like Heather says, it can be a crutch. However, like Lestat says, some people are responsible enough around weed, and have no problems with it at all.
I know for a fact that my parents smoked when I was a child. They loved me, cared for me, in all honesty they were the best parents I could ever ask for. They provided me with everything I could ever ask for as a child, teen, and young adult. Now of course, my parents weren't reckless with it, and were the ones in control of the weed, not the other way around. They also still manage a successful dental practice and my dad teaches courses on TMJ pain at the local dental school.
I was first introduced to weed, in 6th grade, by my brother, who also turned out to be a doctor. I've never tried a harder drug than pot nor do I have any desire too.
Personally, I would way rather get stoned than drunk and if, god forbid, I ever had a loved one with an addiction problem, I would rather their addiction be to MJ over -OH.
When people use drugs(or anything) to fill a void in their life, then problems can arise.