It is common to port your number from one carrier to another.
Porting to GV is just another option and could come in handy for some. I actually am trying to figure out if I can use it in a few months. I have a family plan and one of the lines will be owner-less yet still under contract. I know someone who has a pay-as-you go phone and wouldn't mind adopting him on my plan. If I can port his number to GV and use the number that is under contract it will work well for everyone. The only stumbling block is how to make his non-smart non-feature phone use GV for outgoing calls and not have the other number show on caller ID. If I can find out how to do that I'm all set