Somehow, I don't think the "worst champion in LoL" would be a frequent (nearly perma) ban across several Elo tiers.
There are a few different ways to play and build Shaco. Some of them can duel, and some can't. It also depends on your team, and the enemy teams. If you try to assassinate or duel with a split-push build, then you're going to get owned hard. If you enter a teamfight at the wrong time, or pick the wrong target, then you're just going to die without accomplishing anything. If you get kills and then don't use the advantage to push an objective (which Shaco is especially good at), then you aren't playing Shaco correctly.
I think the reason you're seeing so many Shacos fail is because they haven't adopted the right mindset for him. Shaco is all about gaining a quick advantage with his kit, and then turning that advantage into objectives (which translate into another advantage). Basically, Shaco isn't reactive, he's proactive. You have to have some idea of what your next move is depending on the outcome of an engagement.
Shaco is beyond the scope of mere champion mechanics. You can have every range, cost, cooldown, and damage of his kit memorized, and have perfect reflexes, and know the sickest moves with him, but if you don't know what to do after you kill someone, then you're truly going to "fall off" in late-game.
Basically, the point of Shaco is to get your TEAM so far ahead so fast that you win before late-game (preferably forcing a /ff at 20).