There's a an interesting little memoir over at Project Gutenberg, written by Robert C Adams in 1879, about his voyage on the Rocket circa 1850. In his last chapter he devotes much space to shanteys, and the shanteyman who leads them:

Great latitude is allowed in the words and the shantyman exercises his own discretion. If he be a man of little comprehension or versatility, he will say the same words over and over, but if he possesses some wit, he will insert a phrase alluding to some peculiarity of the ship, or event of the time, which will cause mouths to open wider and eyes to roll gleefully, while a lively pull follows that rouses the sheet home and elicits the mate's order "Belay!" A good shantyman is highly prized, both by officers and crew. His leadership saves many a dry pull, and his vocal effort is believed to secure so much physical force, that he is sometimes allowed to spare his own exertions and reserve all his energies for the inspiriting shanty.

Now, Adams was likely responding at least somewhat to Richard Henry Dana's Two Years Before the Mast, a classic which recounted a brutal voyage under a brutal captain. Adams was a pious Christian who believed that respect for his men would get him better results than beating on them, and so he would be expected to approve of shanties. But still- even in his recounting, you can tell a shantey was something sung to pace the men in a task, not something to be sung for edification in its own right. The sailors apparently were fond of much longer, sentimental songs for that.