Two fold.

The carriers want WiFi to support UMA (search on UMA). This addresses the compaint that "service is great but doesn't work at work/home". This isn't "free" like skype, but it does address some problems WRT coverage. This would be a great feature for T-Mo in the United States, for example.

Users want WiFi so they don't have to pay exhorbant data rates and leech off of existing WiFi spots (coffee house, home, school, work, library, etc). This would be great for college students, for example.