There may be certain panels which only make sense to show to logged in users (active differentials for example) and certain panels which may only be shown to members of certain projects, or administrators, etc.
It would be very nice to be able to put these panels onto a dashboard installed globally, but with the panels having a more private policy without making the whole dashboard private.
(if this isn't implemented, there should at least be a warning saying something like "panel policy makes the dashboard policy more restrictive" - it was hard to figure out why the dashboard wasn't showing for logged-out users)