The CB18 is a great cable but isn't a must. The DC03PRO is excellent on its own and surprised me with the sound. Then with the side volume controls, super easy to plug and play with my phone or computer. The CB18 is a nice plus if you want it. I haven't heard anything that can compare to the sound at 5 times the price, and I have tried.
Headphone Reviews and Discussion - Head-Fi.org
However, I also believe (because I have also the dc03 pro and i can compare directly my other dongles) that it probably needs this 39$ cable to deploy all its virtues but this cable is not very cheap like the dongle. I might get it at some point. I suspect the device sounds much better with this cable and right now I am not getting its full potential.
In any case, is worth keeping the dc03 pro only even only for its discrete steps and buttons. Only for this is well worth having it. I just wish the extra cable was a bit cheaper.