I hate never rez mercys. JUST USE IT FFS
The only time a Mercy should use res is when:
1) A key member of the team has died and you KNOW the battle will fall apart without them
-or-
2) When the whole team has been wiped
This "don't hold on to your ult" talk doesn't really hold true for supports, except Ana and Symmetra sorta. If you ult at the wrong time as Lucio, Zen, and Mercy you delayed the enemy team slightly but it's not worth risking these ults in random situations as to potentially use them and successfully hold or capture an objective.
Symmetra is definitely in a tough spot, I feel she is a bit understated but not to the point where I always feel confident having her on my team. Her abilities are good for first point defense and not much else, not even 'good' just competent. Mei and Junkrat get picked more at these points because they are just
better. It just boils down to her not really being a 'support' and more or less a barebones defense. Mei and Junkrat can delay enemies for some time, and potentially hold them entirely, but Symmetra's setup is just too easily countered.
Glad that Blizzard is looking into making her stronger, and across all modes too. Hope she can stay support too.