As mentioned in the comments if you are heading into a battle situation you would want your medical teams fully briefed and prepared to accept casualties so its perfectly logical they would be at the briefing.
In the actual battle above Endor we do actually see the medical frigate engaged as part of the battle.
In this clip right at the start, you can see the engines of the medical frigate in the centre left of screen , then at two minutes and fifty seconds Wedge Antillies clearly states "They're heading for the medical frigate" as he pursues some Tie Fighters. Three Tie fighters fly towards the ship and an X wing and the Millennium Falcon follow them, chasing them off. The Falcon then flies directly under the medical frigate.
So we can see that the medical ship was committed to the battle.
Additional information on this particular spacecraft can be found on the Starwars.com website
where it is listed as a Nebulon-B Medical Frigate and armed with turbo lasers and tractor beams. They were assigned to escort and protect rebel convoys. In the all out attack on the second Death Star this ship was also committed to the battle. So we see it is essential for medical droids who would have been crewing this ship to be at the briefing.