Roaming is not the problem with 2.3-2.6GHz spectrum now that devices can be made capable of covering the various bands used for BB extension around the world. However, that is describing the availability of international spectrum. Note: some countries have 2.3GHz, 2.5, or 2.6GHz in various bandings.. the suppliers have devised components and devices to work across the various bands so that a device that works in S. Korea's 2.3GHz can be made that also works in Clearwire's 2.5.-6GHz spectrum.
To clarify: It could have been said five+ years ago that the 2.3-6GHz band would be great for roaming because it has been available in most parts of the world. However, until it becomes widely deployed and populated by users, roaming is a goal rather than a revenue source.
Clearwire has achieved about 0.3% of USA population as direct subscribers, and as much as 4% as Sprint wholesale 3G+4G subscribers who might travel overseas to roam on the 4G network where there is WiMAX coverage. That will improve with LTE but will take time.