The whole point is that you can use a solar powered laser based in our solar system to deliver a higher flux to the sail- a flux that wont fall off as the inverse square of the distance of the sail from the source...

If you try to use a photon drive on your ship you have to carry huge amounts of fuel, or if you use solar power it becomes nearly useless as you move away from the sun...

Sure it would suck if congress voted to stop paying to project your power when you only a light year away, but it is the most efficient way... (Actually this very concept, including congress turning out the light was in a very good sci fi story... I cant recall it right now, nor can I recall the author... I _think_ it was Rocheworld by R. Forward, but I am not sure.)