Over 330,000 United/American passengers arrived or departed via SBP during the 2016 calendar year. So yes, some people actually do use it.
Granted, this is an average of fewer than 1,000 passengers per day, which is small by airport standards. But it’s large enough to sustain major airline service via regional jets.
http://www.sloairport.com/wp-content/uploads/2017/01/December-2016.pdf