To be fair, these have been pretty massive growing operations, i read about one with some 7000 plants in about 4 houses, which no doubt went into funding some sort of organised crime. Even if growing was de-criminalised it would prevent organisations gaining funds this way which could lead to more rescources pumped into preventing trafficking and sale of more harmful drugs. Heres a little graph i found (on bbc news) from an article discussing the benefits of re-classifying drugs due to thier harm. Of course this would never happen as alcohol would most likely have to be deemed a Class A drug, which blatantly will never happen.