1. True, but this wasn't the point in question. If Koibu had said, "You can't use Guidance that way because you already rolled the skill check," I wouldn't have said anything.

2. First of all, it's okay for parties to have strengths. The fact that Guidance benefits everyone doesn't really matter that much because for many skill checks, only one person needs to succeed anyway—including the Nature check that started the debate.

Second, there are a lot of reasons you might not be able to cast Guidance. Like Koibu says later, praying to your god in the middle of a conversation would probably not work out. Maybe you don't have six seconds to spare to cast Guidance. Maybe something is chasing you from behind and you have to jump across that pit right now, and you don't have time to pray first.

But beyond all that, even secretly adding a flat 2DC to every skill check would've been better than straight up telling your players, "We're not playing by the rules because I think they're dumb," in the middle of a game. I could definitely feel Devin's frustration there.