Without AF assist ST-e3 is useless for me. Don't see any reason to buy it.
Same here. I have 4 600's, because I do fair amount of in-studio portraiture with two lights on the subject and one on the background. (The 4th 600 is the non-firing master.) That said, the camera / master flash are tripod-mounted, so weight and balance are non-issues.
I would only consider purchasing an ST-E3-RT if I could get it for a "steal."