As if having Isaiah just once in the standard works wasn't boring enough.
Joseph Smith apparently just went through Isaiah chapter 50 and reworked some of the punctuation, changed some wording here and there, and added a little bit of unnecessary emphasis (because Isaiah is already repetitive enough) on a few points. The only significant difference between the two chapters is an omission in verse ten of some advice to "trust in the name of the Lord." I'm not sure why Smith omitted that, considering it might be the most straightforward teaching in the chapter, but it doesn't radically change any kind of doctrine. The two chapters are still pretty much the same conceptually, tonally, and doctrinally.
Maybe his reasoning for siphoning off so much Isaiah by this point is becoming less about building credibility for his manuscript as the word of God and more about filler. If it were possible to make a movie that was to be considered direct revelation of God, this chapter would be the equivalent of a massive explosion going off repeatedly from various camera angles, followed by the protagonist walking out of the blast radius in slow motion. He'd put on a pair of sunglasses, raise his gun, and exchange a few meaningful glares with the antagonist--all still in slow motion. Except at least that kind of filler would still be kind of fun to watch.
So maybe it's more like a remake of a classic film--let's use Gone With the Wind as an example. In Joseph Smith's remake of Gone With the Wind, he'd periodically utilize footage from the original movie as some kind of flashback material, cutting these scenes in at odd intervals until he'd gotten his remake to come reasonably close to the almost-four-hour running time of the classic. The Bible is huge, and it covers thousands of years of history. If Joseph Smith had come up with his own book of scripture spanning more than a thousand years of history and it came to a measly 150 pages...maybe he was worried he'd get laughed at. How can you take a book claiming to be a second Bible seriously if it's hardly the size of a decent novel?
Hence the filler.