Getting too many body types out there, without supporting lenses, doesn't help sales all that much. Look at the NEX line for instance, or Samsung. Lack of lens choices has limited the growth and acceptance of those systems to a greater or lesser extent.
For Fuji, starting a new line of full frame bodies and diverting their lens production into it as well reduces their ability to grow a reasonable lens lineup for both systems. From what I read it appears that they have done a good job at putting out only a few higher quality offerings lens-wise, while steadily evolving the system as interest builds.
One of the reasons micro four thirds has had the unlikely growth it has is because of the strong lens support. Thom Hogan calls this a "chicken and the egg problem". There's no use designing and manufacturing lenses if nobody buys enough bodies; but nobody will buy enough bodies unless the lens choices are there.
Now, if they do a fixed lens to compete with Sony for instance, that's a different deal entirely. Then you don't have to engineer a whole new system of lenses.
To me, the final problem with full frame compacts with interchangeable lenses is the size of the lenses. If you put leaf shutters in, put the lens very close to the sensor, and use just primes you can be very small and still be fast. Lili posted about that showing an old film compact Olympus with an f/2.8 lens. But if you do focal plane shutters and want fast wide-range zooms the lenses will be as big as DSLR's. This kind of makes "compact full frame" an oxymoron in that case, lol.