I think Jump kicks function like Uppercuts, you can only finish a combo with them.

Disclaimer: raidenthefridge is only guessing CALM THE FUCK DOWN.

Im pretty sure you can start kombos like this. The Jump Kick Lands then you immediately do sai blast or gas blast. The projectile is actually an air projectile a few inches off the ground.

you can start combos with them

i usually wait till i hit them in their lower legs

You can use "deep" jump kicks to start some combos but you're not going to get much with most characters unless they have something really instant or OTG, like Ermac's Push / Slam and even that will probably require some corner help.

You're way better off trying a jump in punch to start combos, as you can buffer any combo off of it hit or block.

Now antiair jump kick (Jump kick right after you see the opponent jump) can net you some good combos, which in most cases can be tacked onto the end of a string that nets a launcher.

Kabal for instance can anti-air j.k, then spin to start / continue a combo. Scorpioncan aa j.k then spear. All because when you use the j.k anti air you get a couple more seconds of hand time.

It should also be noted that most air special moves will connect after a jump kick. Kabal's air fireball is a bad example, but all the air grabs (Sonya, Jax, Kano) are free after a jump kick, all the teleports (Smoke, Scorpion, Ermac), all the angled air fireballs (Sindel, Ermac) and all the dive kicks (Kung, Sonya, CSZ).