I've heard many people proclaim that photoshop is superior to the Gimp because Gimp only supports 8-bits colors. The same people often have their digital camera set to store their pictures in jpeg, which is 8 bit. Nobody I know with a compact digital camera has their camera set to save the photos in anything else than jpeg. Many people I know with digital SLRs still has their cameras set to store in jpeg. The point here is that jpeg is 8-bit per channel. Unless your shooting in tiff or raw, there is no need for 16-bit channels.
Currently I rarely use 12/16bit channel photo editing. The reason is that it's to slow (ok I admit, my computer is 3 years old, but hey! I use linux, haven't had the need to upgrade for three years, and I'll probably use my computers for 2-3 years more untill next upgrade). I convert my RAW files to 8 bit channels with Ufraw. In the raw conversion I adjust colors, white balance and exposure, then I only use Gimp to do minor changes, cropping and the like.
What I really would like, is adjustment layers. That I miss. CMYK is really a non issue unless one need real color proofing and do proffesional work.