The market for combining Wi-Fi and cellular on a single device is one that hasn't quite been worked out yet: For cellular operators without their own Wi-Fi plays, a combined device might not pay off and could cannibalize their own cellular data networks. Technology Review Magazine suggests that cellular operators may view the combined devices as sort of a tease, to get users hooked on the idea of higher-speed wireless data just in time for them to start unrolling more ubiquitous and higher-speed cellular data offerings.
The catch will always be cost so there's a chance the tease may backfire. If users get hooked on Wi-Fi networks that are free to access, they may decide to go out of their way to find a free hotspot rather than pay for the cellular access which at least these days is far more expensive. However, it's likely that a certain market segment will pay for the convenience of having the higher speed wireless data from the cellular operators in more locations.