I find it easy to control a party of 6 with RTwP. I can tell all of them what to do in no time. But i'm able to do it all at once. With turn-based, even doing only 4 is a drag, because i have to do them one at a time and watch each of them carry out their orders before i can move to the next. Maybe allow me to start giving orders to the 2nd party member while the first one is carrying out his attack? I don't know...in the single player fights i just get bogged down and my attention starts to drift, especially in big fights.
Watching them carry out their orders isn't even the major issue since you generally want to see if your command was successful or not. The major issue is having to wait for the AI to go about their turns, at which point I'm already on another monitor browsing the web or watching a stream.