I haven't time to read this whole thread, but do feel strongly on this issue, so here goes.
Taking ANYTHING which impairs your ability to act responsibly while caring for children is not responsible, is not good parenting.
That includes "getting shitfaced (horrid phrase)" on alcohol, cannabis or harder drugs.
I enjoy a drink, but can honestly say that I have never been drunk in the 8 years since ds was born. I am a parent, it is my responsibility to be able to deal with any circumstances which may arise to harm/upset my son.
Furthermore, cannabis, coke etc are ILLEGAL. Now whatever your views on whether cannabis causes serious damage or not, whether you class yourself as an occasional 'recreational' user, or an addict is immaterial. You should NOT indulge in illegal activities. The law applies to all of us. Just cos you may not agree with it does not give you licence to ignore it. And responsible parenting includes being generally law abiding (otherwise how do you teach your children to have respect/regard for any rules, including yours?).
I knew a family where the parents were habitual users of cannabis, 5 to 7 days a week, every week. By the end of each evening, they were incapable of looking after themselves, never mind their 4 children (2 of whom were toddlers, who suffered epilepsy).
So to baldly answer the opening question
Yes, taking drugs and being a good parent ARE mutually exclusive.
It's not just YOUR life you are fucking up.