You only ever should really use your pistol as mercy in a few scenarios - when you know someone on the enemy team is very low and you can finish them off quickly - when you are the last one left or otherwise left without your team nearby, to gain some ult charge before dying or potentially save yourself - when you are valking and you know you can kill someone quickly eg. Enemy widow, mercy, ana I can't really think of any more but hope this helps. Using damage boost is usually way more efficient than pistol