Showing an audience an idea in an unfinished website may not be a big risk.
There are likely to be many future developments that only exist in the minds of the founders.
Furthermore, there are many developments that can only evolve from a certain set of initial conditions that are unique to your website. These are only likely to be released to the public progressively once the website is launched and people start using it.
A website is a complex beast.
There aren't many websites around with just that one single feature that can make or break them (if copied by someone else).