Is it bad practice to use multi-class over multi-label classification?
Perhaps I understood the question wrong, but multi-label and multi-class problems are fundamentally different, as the multi-class is exclusive (no instance can be classified into two classes a the same time), while the multi-label relaxes this condition. As Wikipedia on multi-label states:


  in the multi-label problem there is no constraint on how many of the classes the instance can be assigned to.


So, using multi-class or multi-label depends on the problem itself and not on how you solve it.