When the feature doesn't exist! For existing workflows/functionality; I think using a new tool for a feature that doesn't (and won't for some time) exist is a _perfectly_ good reason to introduce the tool. On top of that you're looking at using something that is mature and well-used, Lucene/Solr.

If you wanted to be cheeky you could go ask your CTO if they would foot the bill for someone like 2nd Quadrant to write CJK full-text search into pgsql. ;)

I've done this for some features/issues in random OSS projects and if you feel someone else has the same problem I always feel paying money to have it implemented is well worth it.