Using RTGs for power gives the rover a ton more flexibility though, and helps make the most use out of the time that it has. I remember from reading the Mars and Me blog (written by one of the rover drivers) that the cyclical nature of solar power put some pretty big constraints on the rovers. It wasn't just a day/night thing, they had to be really careful about the orientation of the rovers' solar panels during the martian winters. A lot of the time they'd just park the rovers for the winter. Plus, the waste heat from the RTGs can be used instead of electric heaters.

I suspect there is an upper limit on the amount of solar energy that can used to power a rover at any given point in time. I'm assuming the nuclear option provides a much stronger power source.