Zoned hvac saves energy if the installer and "ALL'' subsiquent service companies 100% understand how the system works. Need i go into detail of what happens with inept contractors messing with it?
I have 2 systems because I have never been able to find anyone savy on zoneing a single system,plus if one go's belly up,we can retreat to other side of house until it's fixed.
Wouldn't all of us like an automobile that gets great fuel milage and easy to park while commuting and running errands all week then pull a 30' camping trailer on weekends? It's called haveing your cake and eating it to.

Have you already done the easy stuff - having effective weather-stripping around all outside doors, double-paned windows, closing up obvious areas of outside air infiltration and beefing up insulation? These things provide the biggest payback opportunities to provide comfort and save on heating.

I too live in the Bay Area and find that our heater doesn't come on overnight other than during the coldest few weeks' worth of overnight temps. Our overnight setback temp is set to the high 50s. By the time we wake up, the thermostat has triggered the heater and the house is nice and warm to start the day.