Lets not oversimplify this, because I love this topic. The benefits of XC will come from the increased mitochondrial density, metabolic and contractile enzymes, and a greater csa of capillary bed within the muscle of type IIa fibers (fast oxidative-glycotic). Another benefit is you will strengthen auxillary muscles in your feet, ankle, legs, and thigh along with there corresponding bones and joints. This tends to lend itself decreasing the chance of injury when your workouts in track become fast. Any extra unwanted weight that you have will be shed during this period as well.
The difference between sprinting and running (oversimplified version here, Mike can and has given a more detailed version somewhere in this forum and he can certainly correct anything I have wrong) is neuro-muscular recruitment of fibers, arm swing, foot strike and contact time. That is basically everything, except the form, and by this I mean knee drive and heel lift. Any race up to a marathon requires a high knee drive and high heel lift throughout the race in order to be ran fast. So what does this mean? Arm swing, foot strike and contact time are functions of the speed at which you are running. The sprinters arms are farther in front and back and don’t cross the body while a distance runner has less forward and backward movement and most cross their bodies. The distance runners heel will strike the ground throughout most of the race, while a sprinters heel will almost never strike the ground. If correctly done both plant somewhere on the ball of their feet. With their heel striking the ground the distance runner has a greater contact time than a sprinter. These create the mechanical differences between sprinters and distance runners. With a greater contact time, the less force and elastic energy you can use, which gives you a shorter stride, which means your arms are not swinging as far out in front or back as a sprinter does. Which leads me to one conclusion, all of this is based on neuro-muscular fiber recruitment. This a learned skill which has to do with increasing myosin ATPase within your muscle fibers and motor-neuron enlargement.
Properly done, XC training will increase the oxidative potential of fast twitch fibers while more importantly increase the myosin ATPase activity within slow twitch fibers (increasing contractile speed). This is why I believe so strongly in XC for sprinters, especially 400 and 800 meter types. I don’t believe in specifically training for energy systems involved in a competition. I train my kids in terms of strength and fiber recruitment as well.
As for distance running hurting form, extensive tempo and continuous tempo are aerobic in nature and they allow you to concentrate heavily on form. I personally find a good distance run improves my sprinting form as does a good high intensity strength-endurance workout.