I also believe the USCG National license difference vs. STCW will diminish over the next few years and within 5 years or so there may no longer be a difference.
As to HELM, first note there are several different leadership courses but this one as taught at MPT is definitely not the same course a business school would teach. I can't speak as to other schools.
I would say it's at attempt to teach some skills that have been shown to be greatly lacking in some major situations around the world. It's not how to run a profitable boat which would be the business school emphasis, but issues that definitely impact safety at sea. It's also to address many of the issues widely seen in the management of commercial and recreational boats, areas in which Captains, Mates, Engineers and Stews have not been trained formally in the past. The objective is to achieve good communication, enhance decision making on board, teach recognition and adjustment to situations, plan and allocate resources, operate in a way to both be effective and follow applicable rules, and learn to assign duties and direct others in an effective way.
I think it's a recognition that there is more to being a Master than knowing how to operate a boat. You depend on others and your ability to lead them. It is a management and leadership job and for many who take this course, it's the first management and leadership training they've ever received. The larger the boat, larger the crew, the more your performance is dependent on the performance of those who work for you, under you, not on just what you do.