I'm pretty sure it's not the "noisiness" but the multitude of voice lines it has. With something like those two other examples, it doesn't matter so much because they aren't exactly going to play during a chaotic fight. On the flipside, if Requiem's voice lines were played to the enemy team, one of like fifteen very different sounds could end up playing at any time, potentially obscuring the main sound effect of the spell, making it a case where someone could have no idea what Karthus is currently doing due to having not heard the voice line before. And after learning what Karthus does, that's still fifteen different voice lines you'd need to develop a mental response to.
Compare that to like Rek's global screech for her ult. There's absolutely no variation to it - if you've heard it once, you'll hear what it'll be like every time afterwards when she uses her ult. Overwatch's heroes used a similar principle for their ultimate voice lines, only have two - and while they're very different, their differences actually serve a vital purpose, given that one always plays for an enemy ( or your own ) ultimate, and one always plays for an allied ultimate.