The question that jumps to mind for me is: are there other missions in planning that require refuelling in space? If it's only Mars missions then having propellant sit around in space for a year or so waiting for launch windows just seems like a waste of infrastructure and an opportunity for gradual wastage of propellant. In that case it would be better to have a BFS tanker that takes more fuel per launch to reduce the number of launches needed for each round of the colonisation fleet.
That's likely it. Elon likes his rockets to be tall and slender, so he might describe rocket that is not tall and not slender as "looking weird."
I'm fully on board for the 'Stumpy the Weird Tanker' being the most likely thing. How blunt can you go with the nose cone on a 9m vehicle?
This is with the same shaped nose as the manned/cargo version.