There is so much fail in this I don't even know where to start.
Marijuana has far more positive health effects than negative. In fact THC the chemical that gets you "high" is anti carcinogenic and has proven cancer fighting effects http://www.ncbi.nlm.nih.gov/pu...
"Purity" of street level product. Oy vey what a statement. You realize your fruits and vegetables have pesticides and herbicides on them as well right? Not to mention all of the nutrient "uptake" from fertilizers is in that plant. You are what you eat, includes plants as well. As for "purity" of the THC content, you either have cheap herb or dank green. Even a plant you grow yourself will have varying quality of THC. In the same way that your vegetables grown in the back yard won't have 100% the same nutritional facts as something grown on a properly fertilized farm.
Finally, PCP laced marijuana? Are you kidding me? Why would any dealer bother lacing weed with PCP? You know PCP is insanely more expensive than marijuana right? Why would a dealer undercut his own prices lacing it with a drug as ridiculous as PCP? Stop quoting DARE literature from 1982.
Want to know a secret about life long tobacco smokers? They're most likely cannabis smokers as well (that anti-carcinogenic effect).