T-Mobile Looks to Redefine the Phone Number with 'Digits'
Seriously, I presume the difference is this would work over the voice network as well as over data.
What if you have multiple devices and multiple numbers you want on every device, for them all to work and call each other in a "round robin" "follow me" type of way? You're lucking out with this, as BETA testing only allows you one of two options - One Number Multiple Devices or Multiple Numbers One Device. And since they are separate, you can't go between trying both.
What incentive is this? It would be easier to have a setting in MyTMobile that says, "Ring all devices selected"