Eh, solar panels are useful in orbit as well. They're particularly useful for avoiding the need for a heavy chemical stage because you can use them to power light weight ion thrusters instead.
On top of that I'm not really convinced they can put enough solar generation on there to drive the thrusters sufficiently, putting aside the additional costs of things like radiating off all the heat generated by the thrusters.
They wouldn't. That's why they're aiming to use plasma thrusters.