From what I've heard, the front licence place in California is for parking meters and hourly/timed parking areas (like max 2-hour parking areas where they come around and chalk your tires). Since most meters end up at the front of the car (either in a front-in, diagonal, or parallel park), the meter patrol doesn't have to walk to the back of the car or walk into traffic to write down your licence plate number when issuing a ticket.

I'm originally from Florida where there is no front plate. When I first came to California 12 years ago, I was doing some work in downtown Sacramento. I had no choice but to park at the meters everyday for $3.00/day. I ended up with an expired meter one day and I got a ticket for the expired meter, and a fix-it ticket for no front plate (I had a personalized airbrushed plate with me & my wife's name on it). So yeah, they will get you for it. I don't think they would take the time to pull you over for it, but you never know with some of these "Gung-Ho" coppers out there.

I don't know if it's legal to just have your front plate in your front window (inside your car). I would love to take that front plate and holder off... I'm sure there's some drag produced there! However it did save my front end from getting f-ed up by some a-hole with a truck who backed into my car pulling out of a parking space. The only damage was to the front licence plate and holder.