And that ignores the fact that not all of those airports are going to have the same airline coverage as the major ones (namely: SFO and LAX) - and even if they do, certainly not with a significant selection of flights, and certainly not at readily-affordable rates. To the average traveler, they might as well not exist; even Amtrak or Greyhound are more viable.
Also, again: weather is still a major factor for any airport.
Absolutely false - for NorCal-SoCal trips, all of the aforementioned airports have plentiful flights at similar cost to SFO and LAX. And weather primarily affects SFO - OAK and SJC rarely have the same problems as SFO.
Also, again: weather is still a major factor for any airport.