Thanks AppleMac*Fit for your response....understand I wasn't trying to flame, just was curious.
I looked at your link. I think the only missing from this is the concept of load on the circuit...his example is not representative of the installation of these stabilizers on a vehicle electrical bus. The capacitor doesn't just charge and stay charged in this application, but is continually charging and discharging at a rate proportional to the load. Also, believe alternators are supplying 3-phase (6 diodes).
Here is a link better explaining the application of capacitors as filters:
Rectifiers with Capacitor-Input Filters
Edit:
I'm not sure what parts are in these voltage stabilizers, but one good form of capacitor input filter combines capacitors with an inductor which dumps the excess ripple to ground leaving nearly pure DC output. These are referred to as pi-filters too.