I don't think it was Isengard that split the fellowship, it was the fact that the one ring corrupted Boromir, and the others would also have been corrupted had they all proceeded.
I think Isengard very well shows how evil (sauron) works and what happens to someone who wants to use the one ring (Saruman)
If Galadriel or Gandalf had accepted the ring, that is what they would have become, only more powerful!
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And with that he lifted high his sword and flames ran down the blade
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