The forgery of ancient coins is particularly developed in countries such as Greece and the former Yugoslavia. Not all fakes are modern - some coins were made in antiquity and put into circulation by forgers. Some of these are so bad it is difficult to see how people were taken in by them. Perhaps they were handed on hurriedly when the person who had been passed a dud saw that it was not legal tender. Perhaps they were for paying uneducated people outside the main area of circulation - the barbarians who lived outside the Roman Empire for example.
Ancient forgeries are definitely worth collecting in their own right.
They should not be confused with emergency issues, when barbarous coins were issued to meet a lack of small change. In Britain, almost every ancient Celtic coin of gold or silver has its plated imitation. The models were probably the Roman Republican denarii which were frequently plated. This was so common that the Roman mint started cutting nicks in the side of the denarii to show they were solid silver. It is to be presumed that people did not scrutinise their coins any more closely than we do today when hurrying to move out of a queue in a busy shop.