True, single-purpose “groups” would add a lot of hierarchy items (though not necessarily double the amount, since many objects, like finger groups, could be siblings of one another), but I think they’d be clearer in purpose than being able to parent one mesh directly to another.
With direct object parenting, groups or null objects, like you mentioned (which, use-wise, would be effectively the same), would probably be necessary, so that you can have siblings without an actual parent mesh.
I think, though, even with direct-mesh parenting, there’s not much need to see, for example, when children are selected but the parent isn’t. At least, not at a glance in a collapsed hierarchy view. I see the checkbox as more of a button than a particularly-informative display. If you’ve opened up and selected some children, the parent’s checkbox is a line. If you click it, it selects the parent and all children. If you click it again, it deselects the parent and all children (though, Nomad doesn’t currently have a way to deselect the current mesh, which I think it either should, or the checkbox icon should change).
Rambling a bit to brainstorm now:
I guess the display method should depend on what it actually means. Like, what does object/parent selection mean? Right now, if I have multiple objects selected, I can only really merge them or transform them. The checkbox currently serves to indicate selection and to allow multiple selection without holding the Smooth button.
Can that be indicated? With hierarchy likely taking the place of Simple Merge, will we be able to sculpt on all child members of a selected parent like we can with Simple-Merged objects? Can we smooth between them (omg that would be nice)?