The canonical use case for a subsearch is to define a filter for the outer search. The subsearch runs and its output is transmogrified (via the format command) into SPL. So the output of a subsearch looks something like:
( ( uid = AAA ) OR ( uid = BBB ) OR ( uid = CCC ) )
The output of the subsearch text-replaces the [ $SUBSEARCH ] part of the outer search. This isn't exactly useful for you "as search results" because it's been transformed. And because of the text-replacement approach, the subsearch MUST run to completion before the outer search can begin.
For some use cases with subsearches - where you're not trying to use a subsearch as a filter for the primary search, you might find the multisearch command useful: