The reason for saying that an interface method declaring to throw the base class Exception is bad practice is because it would in fact make throwing an instance of that very base class a legal implementation of the interface, which is what you clearly want to avoid.
This is why you would only use specialized/custom exception types in your signatures.
I wouldn't go as far as saying that an interface should not declare a throwing of any Exception in advance, as someone said in his answer previously, because the interface is in fact not free floating.
You, as the one declaring the interface, invoke or use the methods in a certain way, and you make assertions on how the methods will be used by your side of the code.
It is in fact a contract that you create. And exceptions are a part of that contract. In fact the one using the interface (not the one implementing it) will be done with his work way before an implementation may exist.
