The phrase "verbal appearance" is genius and really summarizes my feelings on grammar. One shouldn't dismiss a person because of his appearance, but one would be wise to consider that he's more likely to be taken seriously if his appearance is up to par. The idea applies just as well to grammar.
Only if you take "appearance is up to par" to mean "fitting in with the crowd" or else you are swapping objective judgements on grammar for objective judgments on clothes and grooming, and begging the question.
Inasmuch as the crowd you choose to fit into can correlate with other behaviors it may be perfectly reasonable to make objective biases based on appearance (verbal or otherwise).
When push comes to shove and you need to actually decide what your assumption is, you're going to assume something, it may at least have the best possible shot at being accurate.
Even Jeff Foxworthy would often joke that he wouldn't want to be operated on by someone who talks like he does.
