Sorry, it seemed obvious to me. I’ll try to explain.
If a ship is going to Mars, it will arrive at the planet with substantial velocity. If the ship plans to park in orbit, it must slow down; otherwise, it will simply fly past the planet. Because using fuel to slow down is less mass-efficient than doing an aerobreaking maneuver (even if you do not plan to land), a heatshield is useful. Aerobreaking without a heatshield is possible, but typically requires fuel and an engine for an initial slow down followed by several months and hundreds of small breaking passes that do not transfer much heat to the vessel (this would possibly delay crew disembarkation).
The BFR ship itself is supposed to do 98% of its slowing down using aerobreaking, so it wont have fuel available to pull itself, or anything else, into a parking orbit (it has just enough fuel to land). Thus, I assume a separate habitat would need a heatshield of its own for breaking into orbit.
Having a station in orbit could definitely have some benefits, but you’d have to be able to resupply it. Nothing is 100% recyclable. Using ISS vapor compression distillation technology only 94.5% of water is recycled; it would take an additional lyophilization system to (theoretically) bring that up to near 98%; and that last 2% would take even more systems adding more mass, spare parts, and complexity.
I wish this were the case. Onboard mechanical systems will ultimately break down, and every micrometeorite that hits leaves permanent damage. Eventually, things will need refurbishment.
I say these things not to tear the idea apart, but rather to suggest that such a plan doesn’t mesh well with the BFR mission design.
The barge/habitat idea fits much better into the “cycler” mission design, where lots of mass, shielding, spare parts, and extra recycling systems wouldn’t be a problem, and where the cycler’s “free return” trajectory allows reuse every time the cycler swings back past Earth (no tug or aerobreaking required).