My Scorpion has become my primary option for bump in the night, I've cleared the property with it several times so far (all animals so far).
Generally speaking, I will always pick the Scorpion over a pistol because A) I am more proficient with it, and B) higher round capacity. I also have a better light on it than any of the pistols.
But the main reason I go with it is simply because I can put more rounds on target more accurately with it. It's that simple.
A better question would, IMHO, be whether to choose the Scorpion over say, an AR15. Most Scorpion owners probably also own an AR15 as well (I've got 3), and I will be honest, I don;t really have a good excuse at this point for choosing the Scorpion over one of them. The AR15 is clearly the more powerful weapon, and I am also very proficient with it, with roughly the same ammo capacity. So why am I choosing the Scorpion now? Sure, the Scorpion with good defensive ammo will take down a BG just fine, but clearly 5.56mm is going to be more sure in that purpose.
I really don't have a good answer to that. In all honesty I think I just like carrying and shooting the Scorpion more now, though I know that's not a good reason to choose it over an AR in home defense situations.