Perhaps Austen wanted to show us something about the nature/nurture argument. Most of the Musgroves turned out reasonably well under the guidance of the senior Musgroves. Perhaps he was sent to sea to get some discipline drummed into him. But even under the mentorship of a good captain (as we believe Wentworth to be), he failed to mature into a decent young man. It happens. Coming from a good family does not guarantee a good outcome in adulthood.