Well, it's not like Mercy is prohibited from shooting or aiming. In certain situations, pulling out the gun helps a lot more than trying to heal people or damage boosting. Knowing when to shoot and when to heal makes her a lot more fun than just trying to be a mindless heal bot. Her free gun reload helps you juggle shooting and healing anyway.
I don't think I'm a Mercy main (I hop between Mercy and Soldier), but I have been playing her a lot more in comp because I usually see people instalocking Soldier and I sometimes don't trust other Mercy players to stay alive. I regularly pull out her gun and get a decent number of kills per match on top of healing.
Someone calls out an enemy's low and trying to run for cover? I pull out the gun and try to help finish him off, and sometimes landing those last hits makes the difference in a team fight. Someone landed Earthshatter or a sleep, but not a lot of people are nearby to follow up? I pull out the gun. An ulting Pharah or Reaper? I won't outheal that, so I pull out the gun to help shut down the ult asap, and probably res to follow up.