The problem with your idea is that means a lot of time spent in the Van Allen belts. Not a good thing for a healthy crew. You want to traverse them as quickly as possible.
It's a tradeoff between the mass of radiation shielding and extra food (and pay!) you need to dawdle through the Van Allen belt versus the mass of fuel you save by taking the high ISP, low thrust route. If the trips are going to be frequent, you'd want to have radiation shielding anyway in case of solar storms.
You could also provide for both, going light and fast on early missions until you can launch crudely processed lunar material for radiation shielding, and then add that to your ships along with the ion drives. The internal core of the ship (the expensive part) thus gets re-used.
Or you could think further on the problem and use the extra mass of cargo you're delivering (with the high-ISP engines) as the radiation shielding on the outbound flight, and use cheaper lunar fuel and no cargo to make a rapid return trip.