Recently I was configuring WHMCS to sell VPS. VPS has subcategories like Linux VPS, Windows VPS, etc. and each category has it's own feature set. I found no way to get all products under VPS Group under one Category. I needed to create different Groups for different categories.
I suppose this problem can be faced by many of the users. So, adding up the sub-grouping can help us a lot. So that Child Group will inherit parent group's attributes and Parent group can widen it's scope of products within its children.