5 amp circuits because of LED lighting
This is an interesting question or whatever. Here's some info.

1) NEC Table 240-.6 (A) lists the circuit breaker trip ratings starting with 15 amps, and NEC Atricle 240.4 (D)(3) tells you you need to connect a #14 AWG conductor to it. Point is your asking why can't we change a standard, and to start the electrical community would have to rewrite a lot of the NEC and then manufacturers would have to embrace it. I would say good luck on trying to get all of that done.

2) Actually reference the last comment that just came up. When you design fixtures just for LED's just like compact fluorescence(see the biax or type PL base lamps), you design them with a specific base that you cannot interchange. Everything else you are looking at are retrofit items that replace an old technology with new. In the meantime we must design lighting loads with the highest rated wattage the fixture will accept whether it is an incandescent or an LED. 

Hope this sheds some light.