A coin with a weak strike will still have luster on the weakly struck areas if it is uncirculated. If the luster is broken on the high points the coin is most likely lightly circulated.
When speaking specifically of Kennedy halves from the first 10-15 years or so, in my experience anyway, this has not been a reliable indicator. While, certainly, worn coins have lost their mint luster in the worn areas, I've seen luster-less weak strikes as well, particularly in the area discussed above in the hair. This is not just my judgement either, I've seen coins slabbed by top tier TPGs as MS coins with no luster in that area. Considering that luster is a direct result of the pressing process, it's certainly within reason that a weaker or less than complete strike could result in less or different luster. Certainly, I would agree that if mint luster IS present, it clearly is NOT wear...but the ABSENCE of luster does not necessarily demand the label of 'lightly circulated' in this particular case, imo.