I fail to see the connection between drinking alcohol in your home and the quality of your parenting.
Grandparent sounds like they might not be a drinker. Fair enough, I have no issue with that, but - as you point out - there is a whole world between being teetotal and being drunk. The stereotype (which, yes, also goes for weed) is that one swig of a drink turns you into a staggering, vomiting wreck. In actuality, the mast majority of drinking (that I have experience of, anyway) is in moderation, with a meal, and results in a pleasant relaxed feeling which is totally unlike being drunk.
Weed, actually, is different - again, IME - in that the effects aren't quite so wide-ranging. Now, I've not experimented with really strong strains/forms, nor smoked an insane amount in one sitting, but a small amount has a very pleasant effect, and - for me - a greater amount doesn't really change it that much. I prefer to smoke in moderation, have a great time, and spend as little as possible.
I have never done anything I've regretted under the influence of marijuana, nor have I injured myself, embarrassed myself in public, or woken up the next day feeling even slightly bad, let alone like I want to stay in bed for a week. Apart from the legal status (here in the UK), MJ is a far superior drug in every way.
My personal opinion is that to prevent party-related mishaps later in life--particularly between the ages of 18 and 22--you need to teach your kids how to party responsibly before they leave your household. Thanks to the keen foresight of federal highway funding bills, this means you will almost invariably have to break the law in order to protect your kids in the US.
An ounce of prevention is worth a pound of not having to visit the cop shop or emergency department--or both--because your kid never learned about the potential dangers of satyrs and maenads before running off and joining the bacchanalia.