Solar power is you sitting 200 feet away from a bonfire. Fusion reactor power is placing a space heater directly in front of you. If you were cold, which would you prefer?
A mirror is orders of magnitude cheaper than any fusion containment system you care to name, also orders of magnitude less mass.
Seriously, do the math, for the applications we're talking about (asteroid mining), given that the solar constant at earths orbit is roughly 1400 Watts/m^2 a relatively modest set of mirrors could be used to heat metal up to working temperature.
