I always travel light and I haven't had problems at USA customs any time. While I've had run-ins with immigration/customs at many places (incl USA), I can't think of a single time it was due to not enough luggage - it is my odd itineraries that draws attention. Admittedly I haven't yet travelled a day trip with no carry on - even if I don't need much I will always bring a bag/small backpack so it doesn't look so bad.

OTOH when I have been stopped I am asked if I have everything and sometimes they express surprise. A simple explanation of packing light to avoid checking bags works every time.