These people are charged. It's crown policy in Ontario that these are proceeded with in all cases.
I think one of the reasons we don't see it that often, and don't see it successfully that often, is that defence relies largely on the credibility of an accused person who has to explain what they did and why they did it. Being found after an accident throwing back 40 ounces of vodka right before the police come is not a very positive way to start out on the credibility front.
I think the limitations of the availability of that defence are practical ones in that if you're drinking to escape liability after some sort of accident or police intervention, it's usually transparent when that is being done for nefarious purposes.