Assuming that you are a high school jumper, I would consider XC a potentially valuable experience. With my sprinters, I generally prefer that they play football or soccer (carefully 
), but XC can be a great option also.
As a typical HS athlete, you are likely doing more than just jumping. If you run anything in the 200m plus range, its valuable to have at least some sort of an aerobic base. Not so much for the racing but to give you the endurance to handle tougher sprint workouts in the winter and spring. Those workouts, in turn will make you a better sprinter/jumper.
As you mentioned, you are primarily concerned with your spring season. If your indoor season is not too important (or if you don’t have one), then as you said, you have plenty of time to get prepared for your sprinting & jumping.
It is very possible to do too much sprint work in one year, so taking some “time off” to build up your endurance can be very beneficial. That said, I would recommend that you do some light-moderate weight training on your off-days and also do your sprint drills (various mach skips, bounding, foot placement, recovery drills, etc, etc) on a daily basis.
Personally, I feel this is a BETTER option than doing non-stop sprints & weights from September through June. The specificity is very imporant, but it is equally important to avoid burnout…both mental & physical.
A lot of high school athlete just want to hit the weights all year. Most are working their “Beach Muscles” more than the sprinting/jumping muscles, and when it comes right down to it, most would rather look good than compete well. Another issue that is hard to grasp for many high school athletes is that they may not be physically mature enough to effectively utilize the muscle mass that they may aquire through extensive heavy lifting. A 17 year old athlete who is 170# of muscle is not necessarily going to be any better if they are 190# of muscle…they might be worse. From personal experience, I see sprinters being able to get away with the muscle gain a lot better than jumpers. Most of my jumpers who have put on a lot of muscle have found they have “lost their hops” to a certain degree. A 25 year old is likely going to handle having a 190# frame better than a 17 year having a 190# frame, so its important to have some patience.
Then again, looking at some “older” athletes, just watch the upcoming Olympic HJ/LJ/TJ competitions. Most of these athletes will be fairly thin compared to their counterparts in the 100m final.
Probably the most important aspect of doing XC…it’s something different, and you might just have fun!!!
