Would you have an example of a type for which it would be useful to have a DeepSeq instance, and that would require a V1 instance? I cannot think of one now; I originaly thought it would be necessary to permit deriving DeepSeq instances for types tagged with "void" types, but as José explained, in that case, the V1 instance isn't needed because those void types don't show up in the representation.
While void datatypes are rare, it just doesn't make sense to exclude them. It's an arbitrary restriction. Here's a constructed example: data X a = C1 Int | C2 a data Z -- empty type Example = X Z We're using Z as a parameter to X in order to exclude the use of the C2 case. Without a V1 case, you cannot use deepSeq on values of type Example. Cheers, Andres