FWIW, 100g is over 30% of the all-up weight of two of my quadcopters. I think very carefully about tens of grams of additional weight for better GPS or FPV antennas...
Granted there is some grey area in the 300-500 size copters, where they are large enough to possibly cause some minor damage to an aircraft, but small enough that the weight of an additional TX would pose an issue.
The registration being proposed by the DOT/FAA isn't going to help with these either; they are quite simply too small for any "tail numbers" to be visible at any distance that matters.
There are 250 class quadcopters that are built for First Person View racing. Races are won and lost because of the difference in a few grams of extra weight.
Any extra weight you add to a device reduces the flight time of the battery.