Use cases:

Free Calls
Make GV your fave and get free phone calls. With The One Ring, I regularly get completely free texting and calls all through my T-Mobile Total Internet plan.

Dynamic Life
My grandfather is in poor health and my parents are taking care of him. My parents were living between two houses, working, and traveling for both family and work. This involved a cell phone each and up to 3 land lines. They could enable/disable phones as needed to fit their needs. Leaving town? disable the land lines to avoid confusing my grandfather.

Data mining
I'm a bit weird and like to have all my data accessible. I've kept emails, im logs, and documents from the late 90s. Its great to have the IMs searchable also.