Mark Kleiman argues that the question matters:
If alcohol is a drug, then 'drug use' is normal, and not all drug use is abuse. That undercuts the entire project of stigmatization underlying much of what passes for 'drug abuse prevention.' If smoking cannabis, snorting cocaine, swallowing MDMA ('ecstasy'), or even injecting heroin, are not different in principle from having a glass of wine, then the moral basis for treating cannabis-smokers, cocaine-snorters, rave-goers, and heroin-injectors as carriers of a deadly plague is called into question, and even suppliers of those drugs might be seen as regulatory violators rather than hostes humani generis (enemies of humankind) the modern incarnation of a legal category that used to cover pirates and slave-traders.
Of course, by any rational standards, alcohol is a drug. What we need is a criterion to distinguish clearly between drugs we allow and drugs we don't. Our current criterion - alcohol good, nicotine fine, 'drugs' bad - is scientifically and socially incoherent.