Ok, so as far as usecases go, files like this one are historically why support for admin-extra.html have always existed in the Solr UI (going back to pre-apache Solr 1.0) ...
<ul style="list-style: outside">
<li><b>PRIMARY CONTACT: <a href="http://intranet.company.com/~joesmith">Joe Smith</a></b></li>
<li><a href="http://intranet.company.com/engineering/docs/search/product-collection">Detailed documentation about the schema design, data flow, and SLA for this collection</a></li>
<li><a href="http://svn.intranet.company.com/search/product-collection">Canonical configs</a>
<li><a href="../products/select?q=foo&facet.field=...">Sample Query</a>
This collection contains all <em>active</em> products that are currently either available for sale, announced for future sale, or recently disconntinued but still listed for SEO (ie: not archived historical pricing data). Documents are updated in real time as principle product data (ie: core fields and editorial reviews) are <a href="http://intranet.company.com/engineering/docs/search/product-collection/abc">published</a>, additional <a href="http://intranet.company.com/engineering/docs/search/product-collection/xyz">batch jobs</a> re-index all products with new popularity metrics from the data wharehousing 4 times a day.
This collection is available for any team that needs to fetch product data using id lookups, or inline basic searches into other contexts -- however any users with a need for deep analytics or heavy faceting should first consult with the primary contact about capacity planning. <em>Any application usage that will result in more then 100 req/sec must be approved by <a href="http://intranet.company.com/engineering/capacity-planning-team">the capacity planning team</a>
I, personally, think there is a lot of value in supporting files like that in Solr.
As far as usecases for admin-extra.menu-bottom.html and admin-extra.menu-top.html ... my recollection is that when the "left nav" was introduced there were people who liked the idea of only including admin-extra.html on the "overview" page for a core (i think previously it was included at the top of every admin page?) and having an easy way for people to inject links into the menu. The primary usecase i remember was for linking to "custom admin screens" generated from Velocity templates (ie: to create a custom search page with specific fields corrisponding to the usecase)