The specific impulse of the fuel is going to be inversely proportional to the square root of the molecular weight of the propellant, which is why they use hydrogen. You can theoretically use air from the atmosphere but the machinery to do that at the sort of speeds required to get into orbit is very heavy, very experimental, produces drag, and doesn't work when you start getting higher. It makes sense for Skylon because the oxygen in a hydrogen-oxygen rocket is most of the weight and the combustion temperature of that mixture is a lot higher than you can currently do with beamed microwaves.