"Parasitic" is a stronger word than I would use, but I tend to agree with moshe. So I'll repeat:
[...] we will find out just how productive they are, how safe of the drivers they are, how good of a job your pot smoking doctors and dentists do, how law abiding the pot smoking citizens are, etc. [...] they don't pull the same equal load in society as non-pot smokers, but they like to enjoy the same benefits and get high, too.
That being said, if cigs and alcohol are legal then marijuana should be as well. I do not believe it is more dangerous. Also, it's criminalization was based on racism not because of it's side effects.