At a guess stemming from ignorance of Elasticsearch, I'm going to assume you'd join PostgreSQL and Elasticsearch like that in situations where you need to perform more complex analysis over the queries to provide results, is this correct? I've seen this particular architecture before and my biggest question was whether or not it was a piece that could be eliminated.
For, say, a tag-based search system (think any forum or booru-style imageboard), is Elasticsearch completely overkill?
I realise I could probably just google it™, but it's a lot easier to understand a product's strengths when they're put in a situational context.