<<Presumably the tolerance would be greater on a lightly loaded/fuelled aircraft...?>>
Maybe, but don't forget another item, what guarantee do you have that the icing is symetrical and not heavier on one wing than the other?? Very easy to reduce your roll margin ! Add an engine failure or a flight control problem and.....
Pilots taking off with iced wings are just taking chances!
It's both unprofessionnal and stupid.