RotamerCalculator is very useful for generating ResidueRotamer objects for an actual Residue, but internally, ResidueRotamer doesn't seem to store any reference to the original Residue. It just gets some coordinate elements from there. It would be ideal to be able to pass a ResidueType and the necessary coordinate elements so that, if you wanted to know the rotamer distribution of a hypothetical mutated residue in a structure, you wouldn't need to mutate in the input structure to do so.
Alternatively, for convenience, the overloaded version could pass the Residue AND a ResidueType. It uses the Residue to get necessary coordinate elements but uses the ResidueType to generate the ResidueRotamer for the residue type.