catwidgets

A facetious example to illustrate the point: I am required to implement DogWidgets and CatWidgets and I have two choices - separate the Widgetness into some kind of base class, or copy-paste.

Now Joe Junior Developer comes along and over a period of several months, is asked to make a series of changes only to CatWidgets; some of these pertain to Widgetness, but Joe is not experienced or familiar enough with the code to understand that, and anyway has not been told to change DogWidget, so we eventually end up with two divergent implementations of Widgetness.

Bug fixes and improvements to Widget now automatically filter down to both (and all future) Widgets; if Widgets need a colour, then even if we only want to give it to CatWidgets at the moment, we can stub it out or provide a default value for DogWidget, and that thought process also forces us to examine our design instead of ignoring DogWidget altogether, which helps to keep the design current and relevant.