For me, I travel a lot to different countries. If I am in contract, my phone will be totally useless when I am out of the US. And if you add up how much you have paid in total at the end of the term, you would find you paid more than what you would have paid off contract.
Even under contract you can pop out the sim and buy pay as you go sim overseas. It's typically cheaper to buy local as your not paying your carriers international fees and then the locals fees.