I just wouldn't, it's 9 months out of your life, why even take the tiniest risk?
That said, people on MN are generally a lot more relaxed about taking all kinds of medications in pregnancy than I am.
Just because there is no evidence to prove a given substance is harmful, that can't really be taken as evidence that it is safe (The absence of evidence is not evidence of absence).
I think we assume that we know all there is to know about the human body, but the reality is that there are an infinite number of things that we don't fully understand.
Unless the risk of not taking something outweighs that of taking it, I will be abstaining. For example, I am currently taking antibiotics, as my UTI presents a proven risk of miscarriage whereas the antibiotics do not. Therefore the benefit to my pregnant outweighs the risk.
Alcohol confers no benefit to a pregnant woman, but might present a risk. I can't see why anyone would risk their baby's health when there is no benefit to be had.