Coming from experience with Solr, the answer is far simpler than the link in the sibling comment indicates: Data is processed and manipulated on import, and it's difficult - sometimes impossible, depending on the field config - to get that data back out in the format it was imported.

Any changes to the schema, such as switching fields between indexed/not indexed/stored/not stored, requires reimporting the data to populate those fields, data which you're not likely to have if it was your primary store.

Here is a very good Quora answer on why you should never use ES as a central repository for data: https://www.quora.com/Why-shouldnt-I-use-ElasticSearch-as-my...