If the fluid has any mileage on it at all, the ugliness of what comes out of the clutch slave cylinder will make you happy you flushed it. I saw a Harley that had water in the clutch fluid and it wasn't pretty. When the bike got hot, the water started boiling in the system and the clutch lever had no effect. We flushed/replaced the clutch fluid and all was well again. It wasn't the Harley's fault, the owner got a good lesson regarding pressure washers and shiny chrome plated reservoir covers.