well, smoking pot can cause DNA damage. just like many of the pollutants in the air can cause that damage. just like many of the foods we eat. just like many of the natural metabolic processes that take place in our bodies every day. for any individual, runner or otherwise, yeah, it's totally about moderation and what/how much it adds to your experience of life. alcohol is one nasty drug. really really incredibly harmful and abused like hell by a lot of people. including runners. including many under-age runners. so, you've got to always keep that one in mind when you're thinking about drug use/abuse.
there's an enormous difference between "needing" to get high and "choosing" to do so occasionally. in the past six years, i've smoked weed 4 or 5 times. with this said, i have no problem with someone who decides to refrain from ingesting alcohol, pot or any other drug. it's a personal choice for every individual to make. they may or may not be missing out on one of the gazillion interesting experiences there are to enjoy in this world. :)