And with a DPDT switch, how would the phone know which user subscription to service? And then there is the potential subsidy lock issue with many carriers. Phone also has to go through a cold reset (turn-off, turn-on) to bond with a SIM. (SIM card is encrypted) When a SIM card is pulled when phone is on, it will either shut itself off or go to a non-subscription camped state.
The dual sim phone actually has two subscription registrations and examines the PCH (paging channel that sets up ring) for both subscriptions.
The cellphone processing works harder, all things being equal, resulting in less standby time on phone.
This feature is popular in China/Taiwan.