Well, I think we left off with that name because the first name does suggest royalty, which Mercer definitely is not, and the second name was meant to remind him of his piratical family, of which he is deeply ashamed (and Sparrow later teases him about when he learns his middle name - there is a whole backstory with that concerning Teague Sparrow ripping off Mercer's family in the past) and drives him to be the obsessed anti-pirate maniac that he is.