Hey Folks! So, you want to convert to Lithium batteries? You want your Prius to be a Plug-in? Well, there may be $5,000 for you in the future, if you live in Florida! Read this: Florida offers money for converting hybrids to electric plug-ins -- South Florida Sun-Sentinel.com
You're asking an excellent question. I agree that the value of a plug-in on an already efficient hybrid system would seem rather negligible. But, this is Florida, the land where catfish walk -- so I'm sure there will be takers. As for me, "If it ain't broke, I ain't gonna fix-it!"
That and better, depending on driving conditions of course. It may be of note that the car is not more efficient, no less energy is used, just the source is different