I think it all depends on where you are from and what you are accustomed too.
In San Diego (north county) Pretty much no matter what day it is I am in either long or short sleeve wicking shirts and bike shorts. But during the Months between Oct-March it can drop from a nice 80f to a cool 40f. Most of you would agree that 40f isn't that bad but when there is a 40 deg drop its pretty significant. I basically will just keep a pair of gloves, tights and a Nylon jacket or Lycra/Fleece shirt with me to layer on top of things. Also keep a beenie under the helmet but I am looking for a better option then that for this year.