@ankara-- It's because not all antibiotics are the same. They all work differently and while some antibiotics can be mixed with alcohol without problems, it can be very dangerous with some other antibiotics. Erythromycin is not one of the antibiotics that poses a major risk when combined with alcohol.
I think if you ask any doctor, they would prefer that you didn't take both together. They would prefer you to finish your antibiotics first. But if a patient insists on drinking moderately while on erythromycin, it can be acceptable.
However, it's important to leave some time in between the two to decrease the interaction. It's also important to prefer light alcohol and not hard liquor, you should eat well before drinking and consume lots of liquids. There should also be someone with you who can help if you feel dizzy or unwell in general.