So? That means absolutely nothing. Simply saying "We've never had one" would have been a stronger argument.
Next person to use the Roost argument is getting locked in a cage with with a paraflinch Jirachi holding a King's Rock.
I agree. It's possible to have a pure Flying-type use Roost right now. :/
It's not like the mechanics for Roost can't be changed in BW to where it just makes them vulnerable to Ground-type attacks.
Or, it could just not learn Roost. Not every Flying-type has wings and/or can learn Roost (Gyarados, Rayquaza, Mantine).
I can see a pure Flying-type being some kind of avatar of wind that is not a bird or have no wings.
One thing to remember is that it took three generations for there to be a pure Ice-type.