You have no idea of what you are NOT receiving.
Well... Since I don't live in CA I don't think that you would really know just what I *might* be missing. However, since I've been living and scanning in CO since the early '70s I *do* know that whatever I *may* be `missing' isn't anything worth listening to in the first place. (When I am mobile I can hear just about everything I need to hear and the *only* reason I may not hear something is that I might not have *enough* scanners with me at that time. Depending upon what time of the year it is I usually have 2 or 3 scanners which pretty much provides plenty coverage.)
One agency I focus on, especially when mobile, is the CHP. They are on VHF low band. That's a fairly large signal (~7 meters) to get in to your vehicle. If everything you're receiving is UHF, that's different; but still no comparison to a good quality external antenna.
Hmmmmm... `Large' signal? `Getting into' a vehicle? Oh, well... Whatever `floats your boat'. I *will* admit that if one is wanting to listen to something out of EBFE *no matter* how far away one is *then* one may need to use some sort of antenna that actually has some gain above `unity'. Though that may also mean that that antenna is probably going to be larger than most people would want to have on whatever vehicle they are driving. (I *have* seen some *supposed* `mag mounts' that come close to qualifiying for that category but, usually those have either been CB antennas or some `whacker's dream' as to what makes their `ride' `sexy'.)
I might also note that I've always monitored CHP mobile frequencies, which are direct from their autos and not transmitted from some high mountain top. Even now that they've added "repeating", I monitor the mobile freqs and often get a better signal than is from the base (I must be close to them - lol).
Like I said at the beginning... You live in CA, I live in CO and I get more than sufficent reception of the various systems and agencies that concern me. (Amazingly enough there actually are times when I either find myself wishing that I could limit my reception or I actually resort to using a `stubbyduck'.)