In all my time when I play Mercy, the only time I ever ‘hid’ was when I knew I was in danger or some angry ninja boi just yelled out loud.
If you expect me, a healer, whose entire mobility and ult that relies on my teammate being dead, to be out in the open just to be killed first before my teammates, thats stupid. If Im out in the open with my teammates where the enemy knows who to target first, I wont be able to use my ult.
To preface again, I never hid nor left my team to die, but I do take cover or at least try to stay out of enemy sights so that I dont die first because youre always the number 1 target.