I am only going to address policies, I addressed users below.
If policies derive from the brand, they never stray too far before people go "HEY! That's not consistent with the brand! (I.e. thats not how I should be treated, that's not how it should be done, etc.)
Different than that, is that brands change over time. From time to time, a new brand statement may be in order. Having a brand doesnt mean it doesnt evolve. In fact, most brands DO evolve over time or they cease to be competitive -- in our world that wouldnt mean market share of dollars/Euros, etc. In our world that would mean a loss of contributors/users/editors/readers.
The policies of a brand can also change over time to respond to an evolving brand.
One of the nice things about Wikipedia is that it is not static. However, in order to have consistency and equity (a key thing for me) there has to be some consistency. Eventually, standing on quicksand wears out even the most intrepid traveler.