I'm seriously not judging anybody who BYOF, but my feeling is that if your dissatisfaction with the food offerings at a restaurant is so great that you have to BYOF, you'd be happier at a different restaurant.
As a concrete hypothetical example (if that's not a contradiction in terms), if I like everything about a place except that the hot sauce on the table is not my favorite hot sauce, I will nevertheless use the restaurant's hot sauce. It's not going to kill me or offend my sensibilities to use my second-favorite hot sauce instead of my favorite. But other people's priorities may be different.
In the case of people with food allergies or dietary restrictions who are dragged to a place not of their own choosing, I see the point of BYOF completely.