In Europe and the developing world unlocked cell phones are common. When countries are smaller, and clustered together on the same continent, travel between them is more common, and unlocked cell phones facilitate changing carriers easily (so that roaming rates don’t get out of control-especially for data!).
You can find unlocked cell phones with 2 slots for SIM cards, facilitating carrier changing for those who travel, as well as for those who want to save money by making only in-network calls.
But to really be efficient, according to those who use unlocked cell phones with multiple SIMs, they need room for more than 2. From the third world comes the idea to build unlocked cell phones with 4 SIM cards. This would eliminate the hassle of changing SIM cards in unlocked cell phones when you change networks frequently.
Of course, unlocked cell phones with multiple SIM cards aren’t available in the US (nor is there any real demand for unlocked cell phones with multiple SIM cards yet), let alone can you easily buy (truly) unlocked cell phones from carriers. But you can find unlocked cell phones that are both new and used—giving you freedom comparable to that found outside the US.