I'm confused. Why did Joseph Smith put verbatim Bible quotes into the Book of Mormon and how does that create harmony whatsoever? From the perspective of someone you are trying to convince that the Book of Mormon is a divine text, it only looks like copying and pasting (especially when the quote is in the EXACT same translation as the popular Bible translation of Joseph Smith's lifetime).
I'm not sure whether you're referring to the passages that are similar to those in Isaiah or to those that are similar to those in Matthew (from Christ's Sermon on the Mount). There is a different reason for the inclusion of each of these.
First, Isaiah. When Lehi's family left Jerusalem at roughly 600 B.C., they took with them copies of the holy scriptures of the Jews, specifically the writings of Isaiah. Isaiah's writings are mentioned by name perhaps two dozen times in the Book of Mormon, and Nephi (Lehi's son) specifically states, "My soul delighteth in the words ofIsaiah
." There is never any attempt made to plagiarize the words; Isaiah is always given credit for them. The Nephites just included his writings in their own sacred record. They aren't actually even identical to the ones found in the Bible, which indicates that their copy may have been slightly different from the copies which survived to be included in today's Bibles. If it had been Joseph Smith's intention to do a simple cut and paste, he sure wasted a lot of his time in making changes. The 21 chapters of Isaiah which are quoted in the Book of Mormon make up only about one third of the book of Isaiah in the Bible and account for less than 3% of the Book of Mormon. When you stop to consider that Isaiah is quoted more often by Jesus, Paul, Peter and John than any other Old Testament prophet, it's not surprising that the Nephite prophets would have also quoted him so extensively.
The verses in 3 Nephi that are close to or identical to those from the Savior's Sermon on the Mount can be explained by the fact that it was Jesus giving the same sermon to His disciples in the New World as He had given to His disciples in the Old World. Of course there are additions to His sermon to the Nephites, things that are not in the Bible. But the fact that He would have given such an important sermon to two groups of people is quite logical. It would almost be surprising if He had not taught the Nephites the same basic truths and He'd been teaching for three years over in the Holy Land.