I have been wanting something like this for ages (wide angle converter that's behind the lens)

http://www.eoshd.com/content/9474/prototype-metabones-speed-booster-equipped-nex-7-vs-full-frame-5d-mark-iii

"On the NEX 7 the camera reports the maximum aperture as F1.3 and it is certainly brighter. The difference in exposure that the Speed Booster brings allows me to shoot at ISO 1600 on the NEX 7 where ISO 3200 is required on the 5D Mark III."

Even then it won't be worth it as Pentax FF lens line up is thin. That's adding a lot of cost to the camera body for little gain. Canon and Nikon, on other hand, have large FF lens line up (including full-frame manual focus Zeiss lenses).

Sticking with the K-mount for mirrorless camera itself makes it the number 1 limiting factor.

And there are others who are using Nex-7 with expensive M mount lenses.

And then there are videographers who are using the E-mount FS100 and FS700 in their professional work (and most of them own Canon lenses -- as most of them are previous owners of 5D Mark II). This adapter (especially the Canon to Nex version) is especially designed for those people, and the price (relatively) is very cheap.

Phillip Bloom is perfect example of type of people who would want this adapter .. he is already covering it on his site

http://philipbloom.net/2013/01/13/speedbooster/

FS100 is pretty close to 5D Mark III in price, and the image quality (for video) is way superior than DSLR. That's the market that would be #1 buyers of this adapter.