I think "etc." is a request to the reader to be a good classifier--simply truncating the list at "etc." is overfitting, and defeats the purpose of the "etc." Contrariwise, construing "etc." to mean "everything else, everywhere" is trying to make do with fewer parameters than you actually need. The proper use of "etc." is to use the training examples to construct a good classifier, and flesh out members of the category by lazy evaluation as needed.

It's not a reasonable presumption that "etc." will cover "any arbitrary thing that happens to make trouble for your counterargument".