I love jab. I think a lot of people try to use it for things it isn't good for which is why it may seem useless. It's also a lot different than the jabs of all the other characters. Most chars use jab to safely link two things that normally wouldn't link (like Mario's jab dsmash or w/e), or they use it to protect themselves from shield grabs after landing an aerial. Marth can't link anything out of his jab, and he can just space to avoid shield grabs, so in that sense it's pretty worthless. I've found much more promise in jabbing people out of jumping approaches to force them into shield. It's just natural for people to shield after they get jabbed out of a SHFFL or something because they can't really move quickly. So they land from the jab and have to make a decision OoS or else you just run up and grab them. The icing on the cake is that jab usually pushes them right into tipper range, so none of their OoS options are usually a threat to you, but you are an extreme threat to them with dtilt, ftilt, fsmash. I would definitely say jab is pretty bad when your opponent is grounded though. Outside of the potential for CCCing, they may shield it in which case you just gave up your pressure as they can safely get away, and because the jab didn't hit, they very well may be able to hit you with an OoS option.
@PP
I don't have any solid Sheik exp so this may not work vs. better players, but I played a Sheik that loved to spam dash attack, and I found that when DDing max distance wasn't really cutting it, it was easier to do if I did an autocancel nair or some other aerial first. I would just autocancel nair out of range, then he would just approach assuming I would stay where I landed, but I would just DD back immediately and catch him. I see M2K do this a lot too, so it should be pretty solid. lol
Can anyone give me a general breakdown for DIing vs. Sheik? Really just dthrow and ftilt... I'm guessing there's no real escape out of most dthrow stuff, but I can always work to minimize the damage. So far I've just learned not to DI above her after a dthrow at ~100% because I get tipper usmashed, but then of course DIing in front of her gets me faired. :|
