This is just pushing the problem one step removed.
Is it not only relevant what you trade for your groceries?
If you can find someone to take BAT for that, in stable exchange rates, then your point stands.
If you have to acquire $ then what the exchange rate is for $ to BAT comes back into relevance.
Again, perpetually trading BAT back and forth for showing ads/watching ads is restating my premise about holding BAT.
Whether people will be willing to do so greatly depends on exchange rates and volatility.