I would not say that what you are saying is a bad idea , otherwise
struts would not been having DespatchAction Class.
Here is what I think about servlets.
Servlets should be used for delegating calls to business logic sitting in different classes or forwarding to views /
jsp's.
It would not be a bad idea to put logic specific to a particular functional module in one servlet.If some major changes to the module happens then a new servlet can be constructed and the previous version may be replaced.
But the class should not grow too large or else maintaining might be difficult in future.
Struts has a similar Action known as DispatchAction for this .