How exactly would it make sense for countries to waste magistrates on a thing that has no real benefit and will only screw those countries who will later take those provinces without a core and get stabhits. Please expalin how it benefits you, I am genuinely curious.
If it gets taken by a non-HRE power, the Emperor can request that it gets returned to you. Of course, he may keep it for himself or the foreigner might not fork it over, but it is some slight insurance.