@Beau Cameron
Hi Beau! You are correct all around. The change in the Lookup list is bound to the lookup key. The key is what is used at the library level and the rest of the data is associated to the key. If a change is made to the lookup list itself (not the key, but any value associated to it) it does get pulled through into the consuming library. Also, as you noted, if we re-index the library, which we are forced to do now, the new value will be picked up for the search. It's just not a very elegant, nor practical solution in the long run.
The reason for the Lookup is that, by having the data local, we do get crawled properties that we can use for search refinement. Very hand in certain business cases and the access to the external data brings in guaranteed data quality. Secondarily, when data changes, it typically changes outside of the content platform. Example: When an employee retires, their retirement data is set in the HRIS system. That event should trigger a number of things in SharePoint automatically, especially from a compliance perspective. Also, name changes, etc. to ensure that the information can be found both by the historical data as well as the current data. The goal is to eliminate stale content. While the document itself may not change, the information about what the content relates to quite often does.
Lastly, we sync the master data with the SharePoint list on a periodic (nightly, weekly, etc.) basis. This ensures that we're always in sync with the source data, now and into the future. The sync process allows us to have local data which gives us the crawled properties and ability to provide strong sorting and filtering for the industries we serve.
