Before the i-phone / andriod era the Sim Lim Sq used to sell also 3 SIM card phones with Chinese hand-writing input. Now I don't see this any more. Good for travel roaming, when you work in more than one city. Stick pre-paid cards inside to call and receive calls. Save $$$ on roaming.
These days PCR phones can be pluged to PC using USB cable and tethering as a USB 3G donggle for your laptop. That means one of the SIMs can be a 3G data SIM. I am look for a feature that is not yet found. That is to pair with more than one set of bluetooth stereo headphone and listen by 2 persons together. To do this needs extra peice of bluetooth chip I think.