As noted in the discussion under the 1 Nephi preface regarding the grammar of “Nephi’s brethren rebelleth against him”, the Book of Mormon text permits the biblical -e(th) ending with plural subjects. In this particular passage in Jacob 5, the use of this inflectional ending is supported by nearby uses of this phraseology (but where the subject is in the singular):
This same use of profiteth is also found in Moroni 7:6, 9 (“it profiteth him nothing”) and in the King James Bible, but again only with singular subjects (“it profiteth a man nothing” in Job 34:9 and “it profiteth me nothing” in 1 Corinthians 13:3).
Here in Jacob 5:35 it is possible that the immediately preceding “the tree profiteth me nothing” led Oliver Cowdery early on in the transmission to accidentally add the -eth ending to the verb profit in the following clause. However, there are so many examples elsewhere of the -e(th) ending being used with plural subjects that its use here in Jacob 5:35 should probably be accepted. In each instance, we basically follow the earliest textual sources in determining whether the biblically sounding -e(th) ending was originally used.
Summary: Restore the inflectional ending -eth in Jacob 5:35 (“the roots thereof profiteth me nothing”).