I quite when I still could physically moderate my drinking. I quite because I realized I had no desire to and all I really wanted to do was drink. If I kept going I knew I would lose that ability. I am not saying that I had control of my drinking at all just that if I wanted to stop at one or two I could. Drinking to me now was getting drunk.
I remember a post about some guy who would go to AA and not understand what they meant by "no mental defense against the first drink". He ended up going back to drinking and the next time he got sober he knew what that meant and it was much harder.
I guess is what I am saying is that this is progressive for most of us and the sooner you figure out you need to quit the better your chances are.