The benefits may be small or large depending on how you look at them. Suppose you need 95% of your launch mass to be propellant for a pure-rocket SSTO design,

The benefits may be small or large depending on how you look at them. Suppose you need 95% of your launch mass to be propellant for a pure-rocket SSTO design, and suppose you only need 85% of your launch mass to be propellant for a hybrid SSTO like Skylon. This seems like a small savings, but if you look at the "dry-weight" instead -- the mass that you put all the way into orbit -- the air-breathing design puts three times as much mass into orbit. This simplifies the design greatly, allowing you to add things like wings, a more robust fuselage, thermal protection tiles, and covers the extra mass-cost of the heavier engines that you need. So you may end up with two designs with about the same launch mass and the same payload to orbit, but one of them is re-usable and one isn't. I'm personally more than a little skeptical that the economics will ever work out, but it's an interesting idea, and it's not completely crazy.