The game has been in beta test stage for age's already, JMM has said there is stuff wrong with the demo that is not, or will not be, wrong in the full game, so as long as that's correct then the full game may as well be released asap - if it still has bugs then we have to wait for a patch to fix them, nothing unusual there from my experience.
From a selfish point of view I'm not bothered about a new demo, I'm not that interested in SP play so a "better" demo is not important to me, but it might be very important be to get others convinced that the full game will work - that would be time well spent if a better demo increased sales.