CELL COVERAGE CAN be fickle. You might get great reception at home but spotty coverage at work or at the gym even though other carriers work fine. And even if your carrier has your entire city pretty well covered, all bets are off when you travel. Sure you can roam on other networks, but your carrier will prioritize its own cell towers, even if there are better ones nearby. And those roaming fees can sure add up quick. It would be nice to be able to switch carriers on the fly, picking whichever one happens to have the best service in your exact location at any given time.