The idea that Sym is sub-optimal as a support on maps like Hollywood or Hanamura A is a fallacy.
She can get her ultimate with one or two well placed orbs which she can then use to add 75 health to her team or she can run through the choke when her team makes their first push to set up a back cap teleport. Add style points for then proceeding to set up turrets along the enemy return path to catch them as they trickle back or jump into the fray from a flank to take out supports and immobile targets.
The "rules" of when a hero is an acceptable pick have been established by the players and are usually incorrect. You can try to rationalize your opinions on one tricks by saying they're gaming the system with SR gains, but that's simply not true when you look at their win rates and other stats.
People seem to have an overwhelming desire to place blame on others rather than examining what they could have done differently. This sometimes applies to one tricks too, rather than reflecting on why they didn't do as well as they should have, they are just as prone to blaming others and thinking their choices and mechanical execution didn't impact the result of the match.
I don't care how you want to justify it, Symmetra isn't a viable one-trick hero.
She's good in niche scenarios, and I do play her in those few cases, but you'll never win the argument that she's actually a valuable hero in all other scenarios.
Obviously defense capture points is her forte. Battle Symm on offense is more for "lulz", trolling, or because the enemy team is garbage and you can easily microwave them.
Am I going to care much if I see a one-trick symm? Nah. The trolling in this game is epic. one-tricks are in almost every game. And even if not one-trick, there are still trolls that will pick the sub-optimal heroes anyways. Just need to play around it and move on. Both teams have to deal with it, so whatever.
Just had a Torb & Symmetra on Gibralter as offense this weekend, we stalled after 1st cap point. We held other team to the choke before 2nd cap. If the Torb/Symm went practically ANYONE else, we probably win that.
Whatever though, just lose and move on.
I don't think it's correct to disallow one-tricking at all, so Im not going to get all fired up over a Torb/Symm whatever, or even a Hanzo/Widow.. I love the Rein/Mercy one-tricks. Personally, I take the advice I've had since LoL and use the "main 2-3 hero" philosophy. Right now I'm in a Reaper/Junkrat/Mei mode.
Junkrat is pretty OP right now, in basically every match on both teams. I like Reaper for tank busting. And Mei is my girl, self heal, stall, control. In a solo world she can control the flow of battle and doesn't tax healers too much.
I do think maining 2-3 characters is the "correct" approach. Unless you're somehow godly at everything you play, which is absolutely rare as fuck and would mean you're rating is probably 4k+. The 2-3 character approach gives just a bit of flexibility while keeping a high skill level on your play. It's the #1 tip on advancing in LoL and I firmly believe it applies here.