Not to mention that showing the UI now builds expectations about "it will have new features X, Y, and Z! ooh I love feature X, I'm in love with feature X!!!" .... and then at the last minute they find out that X has such a horrendous bug that they have to pull back on that feature.

But, doing so causes all of the people who fell in love with feature X to become disappointed with the final release ... even though they never had feature X in their hands, and they're still getting new features Y and Z. So, showing feature X before the product was ready only served to create disappointment in the final product ... no matter how great the product was, otherwise.

The more you release about the final product, before you're ready, the more you're committing to the form of that final product ... before you've actually finalized it. That's basically painting yourself into a corner. Then any changes you want to make become PR issues, instead of internal engineering/marketing refinements.

That actually goes for the hardware too.