I'm a craftaholic and have all 10 retainers doing mining/botany on my main.

Outside of going for a minion I use them strictly for crystal (fire crystals, wind crystals, etc.) farming every 40 minutes. Geared up they can bring back 120 crystals each. That's 1200 crystals per run total and saves me a MINT having to purchase the crystals for crafting. That, and can sell off the extra for a tidy profit.

Cost a bit to gear them up and do some melding on their gear, but getting 120 crystals per run is huge for me.