Hi Damon,
Navigation manager was written to manage and render navigation. Just like a database or lookup table, you want to keep it as small as possible. For example, a site with 2000 pages, not all 2000 pages are navigational pages whose links are outputed in a navigation area, so there is no reason to have 2000 pages in navigation manager; and when it is time to output a navigation, 75% (assume 75% of the 2000 pages are to be skipped) of the rendering operations are spent on if (dont show in nav) skip. It is a major performance issue as I have seen in many customer's projects. I have seen publication, SmartEdit view and page preview performance increase of 30%-50% just by removing those article or new pages out of navigation manager.
Also, as the size of navigation structure grows, and more not yet trained users get into the project, navigation pages begin to get connected to multiple site sections, and that is when you start getting bug tickets like "a site section just, out of its own, disappeared in A...couple of hours later, I found it in B...WHY DOES IT DO THIS? I AM SCARED." Then you will be forced to try to NUKE button (reset navigation index), which is a nice thing to do on a project with a small navigation structure (300-500 pages no problem and no uh oh moments).
"I have another similar issue where I would like to show other items in a
specific list that could be solved with 1 renter tag, as opposed to
making a few containers and referencing them."
A list can pull element data from follow page, similar to the foreach rendertag, just without the memory cost and known issues. Unless you are try to pull data from grandchildren pages, but there are also native blockmark solutions.
Best regards,
-Jian