According to the WSJ, 6 million people per year travel between LA & SF via air every year. I can't find highway numbers, but I gotta believe it's about the same order of magnitude, if not higher.
Improve BART, VTA light rail and CalTrain (in the Bay Area), and I suspect it will have a much larger positive impact on the lives of more people.
All of the above need to be done, both will have a positive impact of even more people.
I don't know, it just doesn't seem like a very compelling argument to me. The state doesn't have unlimited resources. Spend the money where it will do the most good.
[1] http://www.hsr.ca.gov/docs/newsroom/fact%20sheets/Good%20for...
[2] most recent month with available statistics, lots more data available here http://traffic-counts.dot.ca.gov/
In highway terms, that's a significant reduction. A reduction on the order of 1-3% (it depends on a number of factors including number of lanes, and distance between exits) can get you from LOS F traffic (stop & go) to LOD C/D traffic (heavy, but moving traffic).