Full frame is important because...I WANT IT!!
Most people want a full frame Pentax because they want it.
It really isn't going to make others take Pentax any more serious than they do already.
Pro photographers, top level, mid level or bottom feeders, are not instantly going to leap to Pentax. Pentax had their best chance of that with the LX but didn't, or couldn't, follow up with a Pro service program.
People will not immediately take better photos just because they have more pixel density or any of the other BS tech reasons they think up.
People will not instantly compose better photos just because they can now use their lenses as they were designed to be used.
All this is just so much irrelevant fluff that is used to confuse the real fact which is; Pentax fans want it. Their current APS-C cameras are already well known for great ergonomics and design, AND awesome image quality. But, that really isn't the issue here.
There is a good chance that I will buy one if it arrives. But what I find most interesting is this. Though I already own and use other manufacturer's full frame digital cameras, my most used digital camera last year was the Q7. That doesn't even qualify as a dSLR! The second most used was the Q. Not too sure what all this means, or that it is even important? Probably only that those two cameras are with me almost everyday so are ready to hand for most of the photo opportunities. I doubt that will change if Pentax builds a full frame because it certainly isn't going to be smaller than a K5, or a K-01.
But...let's not let logic interfere. I want it therefore I'll buy it!