Honestly, I think the term functioning alcoholic is a complete oxymoron.
Yes, I agree with that, but we all slide into the abyss at different rates and with different styles. I didn't go chronic until my mid-40's.
But in retrospect, drinking seriously affected my life even before I went chronic.
I forgave myself for being too hungover to go to work or I went to work too hungover to work properly, I partied instead of making good grades in college, I risked VD/pregnancy/violence with unsafe drunk sex, I consistantly chose drinking buddies instead of real friends, I was constantly drawn to people and places who drank abnormally.
Was I "functional", yes, I suppose. But look at the term functional: it means it suffices, is "okay", passes the minimal requirements. It doesn't mean excell, or do above average or do great things or reach your full potential.
I look back at all my years of functional drinking and see how it negatively affected my sense of ambition and drive.