"He who marches out of step hears another drum." ~ Ken Kesey
I know exactly squat about cmyk but I imagine if you aren't color calibrated from file to monitor to printer to book you bump into problems.
The pantone bridge is nothing more than pantone color on the left and what that pantone will look like on coated "paper" stock on the right if out putted in CMYK and not converted in your software. That is if you print on a offset rip out of current (or current software for when the Pantone book came out) One reason to keep up on new books is the changes they make in the software (like Photoshop and illustrator or even the rip software) the translation from Pantone color to CMYK makeup can change from the older versions to the new version. Not many change but I have seen some. They always say to keep up on them because of the sun fading or the ink being changed by the paper stock. I have compared old ones to new ones and the color is very close. When printing on offset presses you should always translate the colors from pantones to CMYK before sending to print. This way you control the translation. If you leave it up the the printer they may translate to a new version or their rip may mess it up altogether. Now the truth about the Pantone books is they are not made for us (screen printers). They are made for paper stock and like everything else there is a ton of variables that play into it. (mainly the pressman they can miss a color by 10 percent and say its your fault) If your trying to color match perfectly it is impossible and your wasting your time. For one your comparing flat coated stock(pantone book) to unprinted ink or printed ink with a different texture or finish.Pantone was created so people could communicate color from one location to another (spec colors to be printed). If you don't have a pantone book then you have no idea what 485c is. If you have a pantone book you can make sure 485c looks like 485c.
Interestingly enough Pantone started out as a printing company. A part time employee began to systemize and organize their inks and pigments and later bought the company which eventually morphed into Pantone.Now you know!