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Shinra is an antagonist from the start of the game. The literal opening scene of the game has you blowing up one of their Mako Reactors because it is sucking the Lifestream out of the planet, and ultimately killing it. They are a clear villain from the outset.
Just because they're trying to destroy Meteor doesn't make them good. The Huge Materia are still a great source of power in Gaia. It's not really wise to let an enemy accrue so much power, even for a good goal, because they have proven themselves to be evil and untrustworthy. Besides, even when they load all the Huge Materia into the rocket and shoot it at Meteor, it doesn't work. Not only is the planet still screwed, but it has no more Huge Materia left in it. It's a lose-lose situation.
They do contemplate it. The entire scene where they let Aerith flow into the water is them contemplating her life. And Cloud's resolve to see the journey through to the end is a direct result of Aerith's death. Even though he doesn't explicitly state it, it's pretty clear that he came to his resolution after reflecting on how she died. He needed to end things with Sephiroth not just for himself, but Aerith too.
The game does reward you with some degree of power with the Bahamut ZERO materia. And three MASTER Materias. So you do use it.
Pull my Devil Trigger!
But what's the point of stopping the evil empire when meteor will destroy everything and you don't have any plan to do anything about it? Honestly, the squabble with Shinra can wait. We can go back to overthrowing fossil freaks after the impending apocalypse is dealt with!
That's the thing - the party is wasting their energy on stopping someone who is actively trying to avert a real crisis for everyone on earth. It really doesn't matter that they're using huge material, since they're only using it to save everyone. They could have all easily dismantled Shinra after the fact! Thwarting their plans when those plans involve saving the world just because "they're villains" makes no sense and was really frustrating, tbh.
This is all the more glaring in the end because they don't save the world through their actual efforts - from their perspective it was almost an accident because they didn't know going into the crater that beating Sephiroth would release Holy. It was more like "well, I guess we're all doomed, but we can try and beat Sephoroth and maybe then we'all get an idea or two! So much better than actually allowing people with the capacity to do so try to physically destroy meteor!"
And don't get me wrong - I still really like FFVII and think it's a good game. But if it came out today, it would be torn to shreds for the nonsense that happens on disc 2.
Cloud literally states their purpose for collecting the Huge Materia: "Hey, Cid! What're you doing!? There are generations of knowledge and wisdom inside the Materia. We're gonna borrow their powers and save the planet from Sephiroth. There's no way that we can lose the Huge Materia. You understand that, right?"
The recurring theme of the game is that the planet, Gaia, is alive. Bugenhagen lets you listen to the cries of the planet from the voices of the lifestream. It's dying as a result of Shinra's actions. They're sucking the lifestream out of the planet and killing it. Since Meteor is about to crash into Gaia, the planet needs all of the strength it can muster. And wasting Huge Materia, which are VAST amounts of knowledge and life, would be detrimental towards the planet being able to protect itself. Not only would wasting the Huge Materia limit Gaia's defenses, but it also might make the planet unwilling to allow the humans to live after carelessly destroying such important resources. That's why stopping Shinra is important, because you want to protect the planet from them so it can defend itself against Meteor. And it's also the only thing you can do right now, because Sephiroth is inaccessible in the Northern Crater.
Shinra doesn't see the Huge Materia as anything but a disposable explosive device. Much like their use of Mako Reactors, they don't care about Gaia's resources. Your party does. That's why you have to stop them.
Pull my Devil Trigger!
Oh, I know what they were trying to say. I just completely disagree, considering the planet was in immediate danger. In such circumstances, some things really do need to take priority, because if it wasn't for what was basically a stroke of luck for the party, there would soon be no planet or knowledge of the Ancients to protect. And considering they really didn't have anybalteranative plan, in my eyes it just comes across as contrived and stupid
That you completely disagree does not make it a plothole. It is just you disagreeing with the game and that's it. The party already harms the planet enough with their own Materia. Making use of a Huge Materia, a manifest of even greater collective of Cetra's knowledge and legacy of the planet for something that by the way might not even work because it was Sephiroth's mere power who controlled Meteor and held back Holy is completely idiotic.
Notice that I never used the term "plot hole", though
It's contrived, but it's not a plot hole. It's frustrating reasoning and I think it could've been done way better, but the plot doesn't fall apart because of it.
They had a choice to either let Shinra take the Huge Materia and use it all in an attempt to destroy Meteor (which wasn't a surefire success), or keep it out of their hands and protect the planet's resources, hoping that when it came time for the Meteor to destroy Gaia it would protect itself. Their actions are consistent with the events of the game in protecting the planet.
Pull my Devil Trigger!
Yup. And once you acknowledge that it's a matter of personal opinion whether you agree with it or not. For me, this stance is completely illogical. But that's just me.
(It was a shorthand for my long-winded argument, okay? )