I think arguments about what the goals of an artist, or art in general, should be (note the should) can only be based on personal opinion, but I don't think such arguments are pointless. Changing the way a person views art can make their experience more (or less) enjoyable, richer, etc.
But once the goals are set, an argument over whether an artist has achieved their goals can be decided by logic and evidence. Assuming that an artist who offers their work to the public actually wants the public to accept and consume it, i.e., look at it, listen to it, read it, maybe even appreciate or pay for it, the art rejected here is 'bad', in the sense that it hasn't served its purpose. Granted, this may not be the goal of every artist, but assuming that they want their work to be consumed by the most people possible, I don't think smearing excrement on canvas is the way to go.