Michael-
For short distances (like 6 feet or less), you may not notice any difference between 'true' video/75 ohm/RG-59 cables and the use of video/right/left for Y/PR/PB as you asked. An easy test would be to just try it, and see if the component looks better than the S-video connection. Certainly, if the audio parts of your cable were causing a problem when used for PR and PB, you could try switching them around (use white for Y and yellow for PB, for example). If there is a problem, you would see a color or sharpness difference just from switching the white and yellow in my example.