I've usually only documented databases at a point in time, when there is this need to a large scale change to the system, or a lot of new developers start working with things. Otherwise is seems that we almost require someone to build their own knowledge as they go along as a way of understanding the system.

If there was some other benefit, and we could easily access documentation, I think it might be more valuable. Especially if we could easily expose things like Extended Properties in places like SSRS where a user could better understand what they are working with. It would be great if a view read back through to the EP of the source table as well and exposed the data there.