Some advantages:
- Better tolerance for deep-discharge, meaning you don't reduce the lifetime of a battery by accidentally leaving the lights on
- Reduced weight, and thus improved fuel economy
- Reduced size, saving space in the engine compartment
- Longer lifetime when only partially charged, allowing keeping the battery slightly below full charge and then quickly charging when engine braking
- No lead, which makes the batteries more environmentally friendly
As well as some disadvantages:
- More likely to start a fire (although LiFePO4 is reasonably safe)
- Higher cost
- Voltage not exactly the same, affecting e.g. halogen bulb lifetime (but this aspect will go away with LED lights)
- Not as well-tested in float charge applications as lead acid
Personally, I believe that the 12 volt system will continue to use 12V lead acid batteries (but perhaps transition from flooded to AGM), but cars will transition through hybrids to plug-in hybrids and the high-voltage batteries will ultimately use LiFePO4. For example, all Toyota's hybrids have a traditional lead-acid battery as well.