While most applications of ion thrust involves long "burns" with either a tiny, slow rotation or none, in the case of this question: Thrust strategy to circularize a standard GTO orbit using ion propulsion? you might want to rock the satellite back and forth every orbit (for a fixed ion engine) to optimize for least time to circularization with a fixed solar panel area. (ion engines are typically powered in real time by the solar panels).
edit: You might be in a hurry to get away from low orbiting debris for example - and depending on space weather - trapped radiation like this:
If you orient a satellite with fixed panels for maximum power, you may wish you could vector the ion thrust. This is just an example of a situation where you might want to do this, it's not the subject of the question, nor the only possible reason.
Charged particles are deflected all the time, naturally and artificially, by either (or both) magnetic and electric fields. One could vary a DC bias on electrostatic deflector plates or even adjust positions of some permanent magnets to steer the thrust without doing (too much) work.
Of course, one could just gimbal the entire engine.
None of these are simple, and they'll surely have design, cost, complexity, risk and performance impact.
My question is: are there any stated plans or research to vector ion thrust?