Re: This seems like a good argument for ion drives
Ion drives unfortunately are pretty useless for the quick thrust you need for orientation and positioning purposes. Ion drives are fantastic for slow but steady acceleration - movement between planets (or Protoplanets in this case) is a perfect case and that's what was used on Dawn. But for orientation it's too little thrust to achieve in any sort of useful time frame the desired results.
To offer an example, your currently doing science pointing instruments at ceres. But you need to change orientation to point the antennas at earth to deliver that science data. With hydrazine thrusters that's work of maybe 15 minutes. Say the transmission takes another 15,and another 15 to get back into science orientation. You've lost at most 45 mins of science time. With ion thrusters you would need at least 2 hour to change orientation in one direction (probably more) and so for the same action you've lost 4 hours 15 mins science days. If you have to do this every day that's 3,5 hours of science time per day lost. The mission will need to be massively longer to account for all of that missing science data. And that means you need more fuel for the ion thrusters, etc, etc. And in the end it just works out way more difficult to do it with ion thrusters. Not to mention the orbital calculations to work out how long you need to apply the ion thrusters for to get into the correct orientation position when you're talking reaction time frames measured in hours. If you under or overshoot with hydrazine, it's a quick fix, just squirt some more with ion you've got another couple of hours to wait.
Hope that explains a bit why we still use chemical thrusters for orientation instead of ion thrusters. :)