Yes, it can get cold enough for layers in the SF Bay area, not to mention our relatively mild temps compared to many areas makes us less tolerant of the cold

A lot is dependent on WHERE in the bay area, but I have had it as cold as 28 when I left my house, and high 30's to low 40's at the start my commute are relatively common in winter. We do get large temperature variations, which means adding and removing layers can be important