I don't like the idea of bulletproof coffee because it legitimizes coffee. Which you shouldn't be dependent on to begin with. The Whole30 coffee manifesto is very clear about that.
If the benefit of bulletproof coffee is to get fat at breakfast, then why not drink something else with a spoonful of fat in it, like nut milk or broth? Why not just take a spoonful of ghee or coconut oil plain? Why coffee? It seems like coffee is being advocated only because people are already dependent on their morning coffee and this accommodates their addiction rather than encouraging them to switch to a new drink.
I'm not saying coffee is evil, just saying bulletproof coffee doesn't make a whole lot of sense.