Power looks to be the issue. A single one of the NSTAR engines used on Dawn uses 2.1-2.3 kW; even with the improved next-generation RTGs this design assumes, you'd need a bunch of them to be able to use ion propulsion.
Don't you usually have a battery you charge using the rtg's, so that you can deliver some real power when you need to do a manouvre?
Not to the best of my knowledge; even with battery backup, ion engines have so little thrust that spacecraft with them use a very different style of orbital maneuvers than craft with chemical engines. This [1] answer on Space Stack Exchange discusses how Dawn entered Ceres orbit using the normal thrust of its ion engines.