For starters, someone who uses Sprint. Sprint's latest phones today still aren't compatible with worldwide standards, last I heard.

Second, just because a phone uses the same system as in Europe/etc, it doesn't mean it supports all the different frequencies used in many other countries.

So if you don't have the right "worldwide frequency capable" phone, even if it's not from Sprint, it still may not work everywhere (or at least work well everywhere) overseas.

Finally, just because you can roam internationally, doesn't mean that you want to pay for roaming internationally. Even without their credit/charge card picking up the tab, many people rent a cell phone for the countries they're visiting because it can often cost way less than roaming iwth your own phone. (Of course, people who have to pay their own Carte Blanche annual fee may not care about roaming costs. )