Best to explain this issue with an example.
If a Full Text Indexed Notes database contains 10k docs and an agent updated 1k documents, the database will show 1000 "unindexed documents" until the next time the index is updated.
During this time these 1000 "unindexed" documents can't be found using the full text search.
Better would be to exclude just the items which have been updated from the search.
Example: why should users of a contact db, not being able to find customers by City, if the status field has been updated ??
If would be different if they searched the status field, the only show the documents which have been full text indexed.
Especially because on a busy server it can take several hours for the index to update the full text index even if the frequency has been set to update immediately.