Almost every carrier story we post has at least a few versions of the following comments—"I would totally use carrier X, but it doesn't work very well where I live," and, "I don't know why everyone is always talking smack about carrier Y, it works great in my area." According to the Wall Street Journal, Google's rumored MVNO could put an end to that by not only supporting both T-Mobile and Sprint, but by switching between the networks automatically depending on which signal is better.
Existing MVNOs that operate on multiple carriers still require you to choose which one you want to use. For example, Straight Talk offers SIMs that connect you to the AT&T or T-Mobile network. You can't just flip back and forth between them because you happen to be in a dead zone. The switching stuff is probably going to be complicated to do and possibly annoying until they get the kinks worked out. If you're loading a web page and your Nova-enabled phone decides to switch to a different network, you might wish you stayed on a regular carrier.
WSJ also notes Sprint has included the option to renegotiate its contract with Google if the service takes off. This is still just rumor until Google announces something, but we've been hearing rumblings of Google's carrier ambitions (code named Nova) for some time now. Who knows if this carrier switching scheme will happen, but Nova needs some sort of hook to have a reason for existing. There are already a dozen MVNOs out there running on the same networks.