Lucas wanted all the main characters together. That's why Luke, Leia, Han, Chewie, Threepio, and R2 are in the shuttle together. There's no real point in bringing droids along on a military attack. The problem is that Han's character arc has already played itself out and he is left with little to do. They could have come up with something, but that would probably have required another story thread in addition to the three that were already going on.