The intent of choosing a subset of county-maintained roads to assign alphanumeric designations was for navigational purposes. If they're not used for this, perhaps bad signage has something to do with it.
it is rare for people to know the number of a county route. I am not sure whether Caltrans's policy is cause or effect, though: do people not know the route numbers because Caltrans advertises them so rarely - or does Caltrans advertise them so rarely because nobody cares?
I do know that the pentagon county route marker has been posted in California since 1958 (and I think the current numbering system was effectively in place by 1970), so it is not a matter of novelty. but I know that the reassurance markers are placed somewhat haphazardly, and the trailblazers are even rarer - especially those on green signs.
I can think of only three freeway/county-route intersections which have the route marker on the approaching green signs. there is one on 5 in Orange County (might be southbound only, actually); there is Jolon Road off 101 (but, see, I do not even remember its number!); and there is S34 (or S32?) on I-8 in the east part of Imperial County. and maybe one or two more which I am forgetting.
do residents of other states frequently use county route numbers colloquially?