the advantage of GSM is that you can swap SIM cards at will if you have an unlocked phone. I always do that instead of getting expensive roaming or expensive international usage added to my line.
CDMA... not so easy. Even with an unlocked phone, you still have to go to a carrier and ask them to register your phone on their network... not feasible.
That's why most of the world has GSM. They travel or have multiple carriers in their countries, and one could easily swap their SIM with a prepaid one on the spot if needed and more cost-efficient.
But again, in most of the world, you can actually own the phone, instead of just getting a loaner from the phone company, like in US. Of course, it costs more upfront, but an unlocked phone is really yours. Here in US, even to get your phone unlocked from a phone company is a pain in the elbow. I mean, has anyone tried to go into an ATT store and ask for an unlocked iPhone ( or any other phone, for that matter)? Go ahead and see what happens.