This restriction in DFDL seems unnecessary, and a number of times the need for this has come up.

Removing this restriction may require more than just removing a check. The grammar may have assumptions that an element is either dfdl:inputValueCalc or dfdl:outputValueCalc in that grammar terms for these may be exclusive (in daffodil.grammar package).

If this restriction is removable, we will raise the removal to the DFDL WG as a correction to the DFDL spec, as part of an experience document around the advanced DFDL features like computed elements, hidden groups, etc.