This drive purports to convert energy to significant force (far more than you can get by firing off the energy directly) without reaction mass. The difference between space and Earth, in your example, is that the Earth has reaction mass. An aircraft uses the atmosphere as reaction mass. A ship uses the water it's in, and so on. In space, you need to bring your own.
So far, the drive is not very efficient (assuming it works at all). I don't trust theoretical projections, because I don't believe it can work with physics as we know it. It may never be useful where there's noticeable amounts of friction. However, we already have useful means of converting energy to motion in air and water and on land, so we don't need it there.
Nuclear reactors are currently an expensive way of powering a ship, compared to burning something. They're extensively used in subs, since they don't consume oxygen and can be used indefinitely at any depth, and the USN uses them in carriers. I'm not completely sure why, but it may have something to do with a lack of exhaust gases that have to be expelled and which can mess up flying conditions.