I used that letter because I was making a slightly different point that time around,in order not to waste everyone's time with a great, long, pointless argument, but if you want evidence of the first argument, that evil is actually beneficial in Tolkien's world, there's a second quote here which reads:
“[Ilúvatar: ] 'Seest thou not how here in this little realm in the Deeps of Time Melkor hath made war upon thy province? He hath bethought him of bitter cold immoderate, and yet hath not destroyed the beauty of thy fountains, nor of my clear pools. Behold the snow, and the cunning work of frost! Melkor hath devised heats and fire without restraint, and hath not dried up thy desire nor utterly quelled the music of the sea. Behold rather the height and glory of the clouds, and the everchanging mists; and listen to the fall of rain upon the Earth! And in these clouds thou art drawn nearer to Manwe, thy friend, whom thou lovest.'
J.R.R. Tolkien, The Silmarillion."
This really is a central theme in Tolkien's stories, that evil exists and is permitted by Eru Illuvatar because attempted acts of rebellion against good actually improve the world. So to use Gower's own example, the fact that Melkor's rule lead to the tale of Beren and Luthien is, in fact, why Melkor was allowed to exist and believe himself to be "rebelling" against Illuvatar. This doesn't mean you ought not fight against evil, of course, (Beren and Luthien were literally attempting to thwart Morgoth, which is the key point of the tale), and in fact fighting against evil is part of the "divine plan" which leads to the world being a better place. This is such a significant theme that literally the whole story of The Lord Of The Rings is decided by this point; Gollum's "selfish" desire to take the Ring for himself is what leads to the Ring being destroyed; yet again, an "evil" action ends up being "beneficial". Further, as if this point needed to be hammered home any further, Tolkien says about the Orcs in Letter 153 "Because by accepting or tolerating their making – necessary to their actual existence – even Orcs would become part of the World, which is God's and ultimately good." I actually have much more evidence than that but I don't think anyone will be convinced who isn't already.
CS Leiws has some thoughts on this same subject in The Great Divorce, but this has gone on far longer than I intended.
This is why I try to argue as little as possible online, because it always ends up with me making some absurdly long and overly detailed response to a very short argument, and then other people start joining in with more and more of these one-line arguments that need in-depth responses and it all ends up being mildly frustrating. In fact I think this is perhaps the most sinister thing about everywhere on the internet, how it manages to rip everything up into tiny little pieces, much like how Saruman the Many Coloured pulls the same trick on light, tearing it apart, or "cutting open the ball to find its bounce", as Tolkien quoted someone else in one of his letters. It's a pointless and ultimately nihlistic enterprise.
