I know a lot of people have mentioned some of the arguments I'm about to talk about, but I want to express them myself either way. First off, huge props to the OU Tiering Council for going outside the box on these suspect tests; it might just be what the tier needs, seeing as how we're stuck in a tough situation. However, I feel like this suspect test in particular seems a little desperate, and it kind of contradicts with the values that were considered important during the Aegislash suspect test. More specifically, it contradicts with the concept of not bringing down broken Pokemon from Ubers to check other, potentially, broken Pokemon. As someone who generally enjoyed the late XY metagame, I think we should try to maintain those values.
At the moment, we have tons of borderline broken Pokemon in the OU tier, but none of them really warrant a ban by themselves. This is what makes teambuilding incredibly hard, becuase there are a huge number of threats, and only so many slots to cover them. Having Giratina-O in the tier would easen teambuilding a whole lot, having it checking like two thirds of the metagame. I still don't think bringing it down would be beneficial, though, becuase it is such a huge force in itself. Being bulkier than most Pokemon, having decent Speed, and decent offenses, it can be played in a similar matter as Aegislash. Additionally, its movepool is huge, and its STABs are incredibly powerful, meaning it can go both defensive and offensive, each version having different "counters". Its access to coverage moves, utility moves and boosting options is also impeccable, with Aura Sphere, Earthquake, Will-O-Wisp, Defog, Dragon Tail, Calm Mind, and Hone Claws being some of the most relevant options. In other words, I think its fair to assume the metagame is going to be wrapped around Giratina-O, if we decide to bring it down. We might have fixed one problem, but in the process, we have created another.
While all playstyles in one way or another benefit from the addition of Giratina-O, I think its going to cause more havoc than it fixes. Offense is going to have an incredibly hard time dealing with defensively inclined versions with Will-O-Wisp and Shadow Sneak. Balance and Stall, on the other hand, are going to have to pack answers to multiple sets, like they were forced to during the Aegislash era. When combined with hazard support, especially Toxic Spikes, there is not a lot stopping the set
Valentine proposed, apart from like Clefable and Toxic Chansey. To be honest, I'd rather see the long, painful process of suspect testing each of the threats that currently cause trouble in the tier (Landorus, Mega Sableye, Mega Altaria, Mega Metagross) than us bringing down broken Ubers to solve any problems. This way we can, potentially, easen teambuilding and lessen the importance of team matchup, without having to force a stale metagame, which is what I think will happen if we decide to bring down Giratina-O.