Prius (2004-09) :: Coolant Storage Tank Assembly Need Replacing?

I got a warning light that the Toyota dealer said indicated that the Coolant Storage Assembly tank had gone bad. It's not leaking, just not working. It's a very expensive fix and in the little bit of research I've done I'm wondering whether it needs to be fixed. My understanding is that it takes warm coolant and stores it in a thermos-like container to stay warm a few days to assist cold starts. However, I live near San Francisco and the weather is very mild here. We might have a few below freezing days in the winter, but for the majority of the year nights are only 50 degrees, and usually only 40 degrees n the winter. So, do I really need this fixed? It seems that since the weather here is so mild it should be fine. BTW, I haven't fixed it since the light went on in January and haven't had any problems starting the car so far.