Another way to think of it is that they already have too many solar panels because the probe has to power itself from Earth orbit to its final orbit near the Su

Another way to think of it is that they already have too many solar panels because the probe has to power itself from Earth orbit to its final orbit near the Sun. So, anything that uses what you've already got (solar panels) with minimum additional mass and risk (the little motors to move the panels mostly behind the heat shield) is a winner.