[po]

It is often advocated as a datastore for logging data... which means (in that case) it's usually the primary datastore but perhaps not mission-critical.

[alrs]

It's a great index for log data.

Spew your log data into a standard syslog server, while also pumping it into Logstash.

Using Elasticsearch as your canonical log storage would be ridiculous.

[rodgerd]

Once you start relying on it to understand the state of whatever it's logging, it's mission-critical.