Yes, one reaseon was the unknown alpha acid and the other reason was to put them in the last couple of minutes of boil to kill any nasties that may be present, such as wild yeasts, insects, etc.
As for dry-hopping into an alcoholic environment, I agree that it will take care of most problems. Although, I don't think that it would be out of the question for you to get an infection even after the presence of alcohol. Therefore, I think that it is prudent to use pellet hops or to take precautions when using fresh or leaf hops, IMO. I don't claim to be an expert on dry-hopping I am just passing on the advice of others that have a proven track record.