Scorpion is good if you're able to play very randomly with him. Most people will have a tendency during matches to fall into certain habits, and if you do that with Scorpion you're very likely going to get punished hard. Since human beings aren't random number generators, there's always going to be that factor of the enemy trying to read you. And you'll find that over the course of a match, since Scorpion only has 4 special moves, the other guy will have an easier time reading you than you have reading him. (never mind that so many players play him that most people know how to adapt - since we're talking tier lists here which means the rating should be given with a good player against another good player in mind, so I think we can ignore people being used to noobish tactics with Scorpion).
Another thing is that both Scorpion's 50/50's against zoners (teleport and hellfire) are very easily punished when you choose the wrong one. His spear is also easily punished with either a projectile or a jump over, which if it does get punished alot during a match you will feel disinclined to use it which means your only combo's are going to come from chaining into the spear rather than starting from it, which makes it very hard to do good damage over the course of a match. However, Scorpion's EX spear has armor and breaks other projectiles, but this still means you have to waste meter everytime you want to get a combo off and if the other guy jumps you miss it. The only redeeming factor is that you catch people with it when they come down after a jump and get in a combo, which is actually very good. His takedown I have nothing against. That's simply a very good move, no questions asked.
And one last point against Scorpion is that he's a high risk/high reward player. Because of this, it's very unlikely that someone will make it far with him in a tournament because eventually the odds get stacked against you. It's simple mathematics. If you play enough players, eventually someone will guess right too many times and you loose.
With that in mind, he gets a
4.5 from me. And yes, I know I have him in my avatar. Sorry, Scorps