Using your shine in the air can be useful and effective if done correctly and in the right situation.
1. If you have just approached with a lazer or aerial (or even a shine) and then you shine on the ground, some players prepare to act after your first shine, so if you shine, then short hop and shine again very quickly, a lot of players will be thrown off-guard by this and will get hit by the second shine, which can lead to combos.
2. If they're under a platform you can do the above mentioned tactic, or just short hop and shine, then jump and waveland on the platform. A lot of times, you will be able to just fall through the platform with an aerial, and a lot of times it will hit.
3. Against players that will often wait for you to land (mostly players that depend on grabbing you when you land, mostly peach, marth, and fox's) say you full jump and then right before you hit the ground, shine and second jump, a lot of times they will (if they've been grabbing you when you land) go in for the grab anyways, as they've developed the habit of being able to against you or others, so you'll be able to fast fall with a d-air or whatever you want and they probably won't have enough time to shield.