All three [Tom, the Balrog, Shelob] possess an independence that places them outside the central moral concern of the story - the destruction of the Ring. Their amorality, like their nonhumanity, reveals them as allegorical principles: Tom of life or nature, Shelob of death or blind appetite, and the Balrog of a central disorder that no creature can withstand.
We could object to Tolkien's inclusion of Bombadil and the two monsters because they are principles rather than personalities. But allegory in a work of this sort need not be an artistic failure. Tolkien does fail with these two, however, not because he chose to dehumanize them, but because he failed to make them interesting. Treebeard, for example, is much more interesting than Tom Bombadil, and the orcs more fearsome than the Balrog.
Although we could not call the adventures with the Balrog and with Shelob dull, they both seem to fail, not in execution but in conception. Tolkien has invented these monsters rather than created them from the raw material of folklore as he did his other creatures. We are unable to believe in the Balrog because we have no foundation either outside the work or in it. Dwarfs, orcs, and elves are familiar enough to most readers to stimulate a response. Other creatures, including hobbits, the Ringwraiths, and the Dark Lord himself are fully developed within the trilogy. Not so with the Balrog. There he is, all of a sudden, whiffling and burbling, a Diabolus ex machina, when the orcs were foe enough. He is not dull, but the excitement is on the surface, and we only half believe Gandalf when he cries, "'Fly! This is a foe beyond any of you.'"
Shelob is better executed than her counterpart, but both episodes are artistically weak. For sheer terror, they are on a level with the invention of dozens of science-fiction writers, but terror is not enough. Nor is the argument that only such supernatural creatures could cause Gandalf's death or Frodo's paralysis, for there is still the feeling that these demons are not real. They are unreal because they are extraneous to the traditional framework of the story. (p. 7)
Thoughts on this, keeping in mind that it was written when only The Hobbit and The Lord of the Rings were published?