Why would anyone use a cellphone rather than a USB phone or the softphone on the PC in this case?

I'm mindful of the potential radiation exposure from using cellphones too much. If I'm going to be at a static location with broadband & Magic Jack in the first place, I might as well use a cordless phone with better speaker phone capabilities or a wired phone and avoid the radiation + battery drain on my cellphone.

Sounds kind of redundant to the magic jack itself.

Also, since a lot of people are migrating to smart phones, for internet access on the go, plus doing texting, I don't see this as that big of a threat to the phone companies.