Quite a few reasons why jab is better than ftilt in quite a few situations. The first is, as you said, covering the ledge. It's pretty good at that. Characters that have upbs that have issues sweet spotting the ledge (like captain falcon) will take 40% or so before being able to recover. It's pretty nasty. Second, it's pretty safe to use on shield. It'll eat up shields and poke, and if it doesn't poke they'll be pushed out and you cancel the jab to hit them with the big hitbox when they drop shield. Third, getting someone caught in jab when they're barely in front of you (so that it takes a long time to get pushed out) will do a ton of damage. A good jab can do 15-30 percent. Pretty good punish.
Jab has IASA frames so buffer actions out of it. The lag doesn't last anywhere near as long as it looks.
What ftilt is good for is spacing with perfect pivots, and using the first hit to combo into shuttle loop at high percents. Other than that it's actually only 4 frames faster lag wise on all three hits and you get a LOT less reward (it's only 10 damage) AND if you use it on shield you're going to eat an fsmash.