He has spoken about this in a number of interviews and the summary is:

1. He tried to give the scions the back seat to see how people would react to new characters being the focus instead.

2. He kept them in the background because some people might be attached to them still.

3. In reality, they can only give significant development to certain main characters, which causes the ones with lesser roles in the story to seem like they weren't used well.

4. Going forward, he's going to just bring along the characters that will actually develop, so some or all of the scions may be left behind in future, if they are not going to get any character development/usage.

I think #4 is the right solution. Also, I would be very happy if the Warrior of Light did just set out on their own adventure.