[The Only Real Estel]

True, but even Gandalf still respected the Steward (though you really had to strain to catch it in the movies), & surely he wouldn't have allowed Gimli to disrespect it by sitting in it. Besides, even though he was in the Houses of Healing, Faramir was techinically the Steward, so it was his chair. It'd be different if the steward had no son & no one knew who the next one would be, but maybe PJ thought it'd be funny to have Gimli sitting in the Steward's chair.
And I'm sure Gimli sitting down wasn't that important; if anyone should be sitting down, it should've been Gandalf, who was fighting all day, & had to take on the Witch King. Gimli fought quite a bit too, but he also had the luxury of sailing around in a boat half the day too...