The Silmarillion Survivor

[the phantom]

And you never know when those Eagles may come in handy.

We don't need Manwe's eagles.

We have Tilion and Arien. They can handle all the up-in-the-air stuff. And later, we'll have Earendil. And if we are really in desperate need of air power, we can have Melkor make a batch of dragons.

he provides firm leadership

What good is Manwe's leadership if he's leading in the wrong direction? Not to mention, he freezes up and goes blank in difficult situations.

Melkor is a better leader. Sure, he's affiliated with evil, but according to the text he has a share of the gifts of all of his fellow Valar. That means that he can somewhat relate to all of them, and can understand what his fellow Valar are capable of, therefore he would be better than anyone at directing them.

And as far as him sowing seeds of discord, he wouldn't be doing that if everyone agreed to follow him, would he?

We need to get rid of Manwe while we have the chance!