Single-click selects objects inside the currently selected group - without need to double-click / hold Cmd
I know I can directly select a non-group object no matter how deeply grouped it is by Command-clicking it. But if I want to select a group that's grouped multiple levels deep, I'm forced to double-click my way down until I reach it.
Double-clicking multiple times in a row has annoying feel of excess effort and sluggishness, probably because the minimum elapsed time the OS needs between double-clicks in order to recognize them as separate events is way too long.
In other products I use (OmniGraffle, Axure), single-clicking an object or a group that's one level down within an already selected group selects that object/group.
Single-clicking your way down the nesting levels is much faster and less crazy-making than double-clicking, especially if you've adjusted the default double-click speed to be faster than normal.
I'm not saying remove the current Command clicking or double-clicking behavior; I'm requesting the addition of the model above.