The primary benefit seems to be the lower cost / mass producibility. An alternative to making launch vehicles reusable, is to make their production automated and inexpensive. This is a lot more practical when they're on the scale of a family car, rather than a container ship.

The only advantage seems to be if you need to get lots of small payloads into very different orbits, for everything else the square-cube law and other economies of scale work against small rockets.