If a penumbra of uncertainty must surround all legal rules, then what seems to follow from this?
Goodness... I haven't thought about Hart since reading him for my comprehensive exams in graduate school...
According to Hart, (in "The Concept of Law") what follows from this is that judges must make law instead of simply "finding" or interpreting existing laws. In theory, judges do not make laws, only legislatures do that (in democratic countries). However, what Hart is pointing out is that essentially every rule has enough uncertainty that judges must make the law.
This fact is clearly seen in such things as US Constitutional law. When the First Amendment says that Congress shall make no law "prohibiting the free exercise" of religion, does that mean that I may sacrifice other people to my god? Does it mean that I may smoke marijuana if I believe that my god tells me to? There is a large penumbra of uncertainty.
But Hart points out that even much clearer laws have a penumbra of uncertainty. He mentions a law stating that there must a person must sign their will for it to be valid. He then goes on to point out all the uncertainties in that law -- what if they only initial it?
So, all laws have this room for interpretation. Because they do, judges must inevitably end up making law.