The main use for this seems to be when a module defining a data type lacks an instance you want, and for some reason you can't get the original module changed.
If you continue this line of reasoning: 1. The only reason a module can not be changed is that it is in some library as opposed to your own code. 2. The only instances that can be derived are for Prelude classes. 3. These classes are already known in library modules, so they could have been derived there. 4. The fact that they are not unnecessarily reduces the usefulness of the library, and could therefore be considered a bug. 5. In fact, looking at the standard libraries, they already provide instances for all relevant Prelude classes. This leaves me to wonder what stand-alone deriving is actually good for, only newtypes? Twan