[HerenIstarion]

It seems that whatever accusations made by critics, JRRT is mostly modern writer. One of the issues, now, raised in modern writing is the problem of power, and its corruption (All power corrupts, and absolute power corrupts absolutely). It would have been highly relevant issue to be addressed by the man of the last century, still more the participant of the WWI and witness of WWII, two 'wars to end all wars', started by power corrupted tyrants. That'd be the reason number one - to explore the impact of power on an individual.

Another reason, mentioned M/B confrontation - if it were Sauron who were entirely to blame, than, firstly, the hint of "Evil as Outside Force" would have been by way stronger than what Christian writer would have wished for, and another critique made by many critics of Tolkien's works - that they are black-n-white, without shades of grey, would have been true. But as it is, even the most 'good' heroes have something to repent of, it is better mirror of the things as they are.

Or, for short - reasons for that may be described as political and theological at the same time.

And all of the above, is of course, personal opinion.