Since I'm not using Lightroom for my asset management, I don't make much use of the pick flags. Even those times when I use Lightroom to process whole shoots on their way into the archive, the extra levels of temporary picking have not been necessary.
If you are using Lightroom fro asset management, then the new sorting tools can be handy, depending on the task at hand.
I agree that a label to indicate "not yet rated" is a good use of that tool.
I do something similar. First thing I do om import is asign all new imports to yellow (uncategorised). Then once I get them catalogued I assign them to purple. This way I can quickly fiind any images I have still to categories.
No particular logic in color selection - just a random choice.
I use the reject flag to identify anything I don't want to do bulk deletes.