I totally agree that shooting one-to-one fight would have been in sharp contrast with what Tolkien had written about the demise of poor Sauron. On the other hand I don't see this particular amendment a grave matter. Aragorn and Gandalf (with essential aid on behalf of Pippin) had done everything to make Sauron believe that Aragorn bore The Ring. Thus, it had not contradict everything else in the plot if Sauron, desperate to have his Ring back, would have gone to Morannon Gates and duelled Aragorn.
What I tend to agree is that Sauron's character was left without any proper development, and an attempt to replace it with some rapid change of images during the last minutes of his screen presence is a very poor substitute. Moreover, transforming him into virtually the Eye left PJ without an opportunity to show him, for instance, present at Gollum's torture as it happens in the book. Apart from PJ's tendency to opt on occasion for more eye-catching (sorry for the pun) but superficial solutions, I have to admit that Tolkien's manner to show Sauron indirectly makes an awful lot of difficulties for any director. One would have to re-create this paranoid feel about Hitler's bunker/Stalin's ever lit window out there, where something had been always going on, that no-one could see but it would define everyone's life no-one could hide; and an almost 'physical' presence of fear and admiration. Honestly, I believe, PJ managed to catch this feel here and there in the movie but it is rather in the horror of Nazgul, or in Gandalf's concerns, or in Galadriel's sorrow than simply in the image of the Eye on the top of the Tower.
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