I don't get it. Various net pundits assert that it's a bad idea to season cast iron with butter, because butter consists of proteins + fat, and the proteins will burn and be incorporated into the polymer mesh. That collective wisdom says that it's quite alright to cook with butter after a pan is well-seasoned, but one should not season a pan with butter.
I'm not expecting you to be a polymer researcher with intricate knowledge of whether burnt proteins will slide off during pan cleaning after cooking with butter, or will be incorporated into the seasoning (but hey, if you are, perfect!).
What I'm asking is this: do you have practical experience using butter as the main source of building up a seasoning? If yes, how good is the seasoning?