The incredible douchbaggery of Cloud and Friends

[Sephiroth]

Would using huge materia as a sort of explosive material drain the planet of energy in the same way as when a person uses the materia? I don't remember the game telling me (or the party) that simply using already existing materia would damage the planet. Is this something that was lost in translation?

That's the "problem" here. They always talk about how taking away the planet's knowledge would eventually kill the planet as it can no longer keep up its rebirth cycle and also not growth this way. It was implied how Materia works in-game and then later continued to flesh out the explanation in case it was not obvious enough. Of course it was never said if "a machine using materia" or "a person using materia" works perfectly the same but it does not change that materia always results in energy being drained from the planet as they do not just get it out of nowhere. Of course another argument to be considered of high value is, if you just blow up this Materia, you also blow up the Ancient's knowledge within that Materia. Also not very hot.

It could still be the lesser of two evils to use the huge materia, and Cloud & Co. wrecks this plan for no good reason except "shinra's doing it and we hate them", with no regard the results of their actions.

But that was exactly what I was talking about. We cannot always just think "oh the lesser of evil" and be fine with it. Sometimes we should be smarter than that. And as said, it was not like Shinra having all those powers would just result in a one-time event. Look at the Sister Ray. As long as they have the ways to suck up planet energy and kill the planet they use it (of course Cloud and the others did not know that the third shot wih the Sister Ray was meant to strengthen Sephiroth but that is not the point). I can see no scenario where Cloud and the others letting them use the Great Materia would be a wise choice. I can see the point of someone having a problem with the entire Materia story with the heroes still using a small portion of it, though. However, I have already written about that, of course. The thing is, Shinra was just a slower Sephiroth at this point. We want to rule the world and make our lives more comfortable? Planetary Energy. For years. We want to defeat a WEAPON? Planetary Energy. We want to destroy a Meteor? Planetary Energy/Getting rid of the Ancient's knowledge. We want to destroy another WEAPON and get rid of a psychic barrier? Planetary Energy. We want to continue being assholes on the future because no one actually stops ups? Planetary Energy. We are the ShinRa Electric Power Company. We own this planet. As said, I do understand that in some desperate situations with absolutely no other hope in sight people consider "bad 1" over "bad 2" but I think for the sake of the story the decision of the heroes was better.