I’d like to know just in general, but also more specifically UCD, UCI, and UCSC.
I don’t have a straight answer for you.
But I can’t imagine them requiring an essay they have no intention of reading. Why give themselves more paperwork to wade through?
This link should give you the information you are seeking. Look at the column on how each UC campus uses the PIQ’s:
http://admission.universityofcalifornia.edu/counselors/files/freshman-admission-matrix.pdf
Some do and some don’t. I know this is vague, it really depends on what else is on your application. Some readers already have made up their score before they get to the essay. With that said, if the reader is on the fence, the essay can/may help you.
My cousin told me for UCSB that each Personal insight question was read twice (heard from UCSB reception). I am assuming that they do read the essays atleast once, but not in much detail.