Having been in the print industry for almost 17 years and spending the first 4 of them prepping files for printing to film, I would have to agree with Sean. The hard part, the part where things could go wrong is pretty much worked out once the press sheets are setup. As long as Sean has seen a high resolution CMYK version of the digital proofs and checked over each channel and the images are clear and crisp, I say go for it! If the prototypes are truly printed through a regular color laser printer (or at that size, probably a large-scale inkjet plotter), and not the actual printing press that the final cards will be run on, then the color matching is pretty much useless. If you don't need the prototype for promo purposes then the possibility to check for color is the only factor that isn't covered by the digital proofs, and that possibility isn't there, unless the USPCC has their prototype printer calibrated meticulously close to their printing presses. Which I would think is highly unlikely.
In the non-playing card industry, approvals from digital proofs are an everyday occurance. We do a lot of work for big alcohol brands like Absolut, Jameson and Malibu and do a lot of large-scale grocery displays for them. We approve prototypes for the displays but they are always "white box" prototypes, finished displays without graphics to check the structure of the piece. Once the graphics are built to the dielines then digital proofs are used for approval purposes. If it's an extremely important or large print run then one of us will be at the printer for a press check when the first press sheets are printed. I don't think the USPCC allows that! The USPCC works out the Finish and Card Stock, so if there is a problem with either of those then that is nothing Sean or the Prototypes would have foreseen and I'm sure they would correct the mistake. If the prototypes were truly printed on the same press, paper stock and finish as the finals then I would say definitely get them but since they aren't it could be just an unneccessary luxury item.
For my cards I was planning on sending the art for the Prototype deck with slight variations for each face card's background pattern. That way I would be able to pick which pattern density looked the best against the background and PIPS and change all of the patterns to the matching opacity level. Once I found out that the prototypes were only printed on an office printer (making my pattern opacity tests useless), I changed them all back to the opacity that looked best on screen and print on my laser printer.
That's just my opinion on it though!
thanks, Randy