Didn't have time until today to do the bounce card with and without plastic card holder test. The exif are 400D manual mode with 50mm F1.8 lens, at s = 1/40s, a =f2.8, flash 580EX set to ETTL-II mode.
Direct Flash
Bounced ceiling without bounce card
Bounced ceiling with white card in plastic card holder
Bounced ceiling with white card WITHOUT plastic card holder
Bounced ceiling with yellow card in plastic card holder
White and red cards in their respective card holder (note, green card is just behind the red one)
Yellow card and green card in their card holder (white card is behind yellow card in holder)
Personally, I see little difference with the test on with and without card holder (but perhaps indeed the card holder is a little harsher) but I don't think it is too unusable. Please feel free to comment
Omnibounce can be used directly, 60 degrees or straight up (90 degrees) and diffuses the flash light in a small enclosed area just above the flash head due to the translucent material.
A bounce card is meant to be used either at 60 degrees or 90 degrees and reflects the flash light forward giving a stronger flash of light towards the subject as opposed to a simple bounced without reflector or diffuser.
actually i think the bounce card is to direct some of the light forward towards the subject
it can create catchlights and fill some of the shadows from a ceiling bounce