Re: Failure version 2.10.6
CMYK management it's not a DTP operation only - if you're a photographer and you're going to print your work using an halftone CMYK printer - you may want to control the process to ensure the best results.
Gimp also lacks many needed features for proficient photo editing, like larger RGB color spaces, and adjustment layers. Even to open RAW files you need to add third party application like DarkTable or RAWTherapee (Photoshop comes with ACR fully integrated).
As it is now, it's really mostly a tool for web image designers who don't need a more powerful tool.
Anyway, making inroads in the prepress industry is not easy, most workflows are based on what became de-facto standards, and you'll need a truly excellent product to have chances. Lacking Pantone support for spot colors when it's a must doesn't help either.
And I don't know how many Gimp developers have the required hardware to develop and test high-end CMYK functionalities...