Dual SIM was very common even when I lived there 10 years ago - I'd never heard of it in the UK. It was driven IIRC by the byzantine roaming tariffs that covered state to state calls, which could get horribly expensive, particularly as incoming calls were charged. While some states cover huge areas, others don't, so crossing state lines is common, and there were also some examples of calls between some carriers not connecting to others in places as they tried to enforce an especially unpleasant lock in on a seemingly random basis. So even in your own state, SIMs from two carriers was sometimes a good idea.
I wish they were more common here. It drives me barmy swapping SIMs going to Germany twice a month for the sake of cost, but having to carry another phone 'just in case' for the other SIM.
Biting the hand that feeds IT
