Sam, welcome to the community, and thanks for posting here!
Here are my thoughts. I encourage other members to also respond with theirs.
>How valuable is it to have a tool that organizes topical / threaded discussions and sub-discussion among employees, and gives priority to posts & comments that are more engaging among employees? Why or why not?
I believe that it is valuable to have a threaded discussion tool. I don't think that prioritizing posts and comments is necessary. The most engaging threads will be obvious by the number of replies made in them and by the number of different members making those replies.
>How do you measure the growth of your knowledge base?
Report on the number of contributions over time, the number of different people making contributions, and the percentage of the target population making contributions.
>How do you measure the usefulness of your knowledge base?
Here are three ways: