It makes sense to use the app icon if a dialog remains on top so you know what the window belongs to, but in the case of the tool dialogs they only show on the app screen so you should know what app your in. I just like clean lines. 
That’s what I thought as I was looking at it… I plan on adding the ability to add other attributes to a group - such as line color, type and width - so you can place objects in a group with a given style if you will. Then when creating a new tool you would be able to select which group an object goes in, automatically styling the object without having to select the color, type, and width everytime. So the groups will do more than just setting an item visible or not. I may at some point redefine the “groups” as “layers” in keeping with the AAMA/DXF format. The groups tool as it stands acts more like layers than it does grouping items.