Like several of the above posters mentioned, this set is likely a good one to clean up late game, after your opponents have been thorughly weakened.
Similarally to, for example Swords Dance Weavile. Weavile already has the speed, but the SD increases the attack. Staraptor has the opposite, he has the attack, but needs the speed.
(I know that weavile has higher stats then Staraptor, I was making a comparison).
The idea of this set is to function as a late game sweeper, not the opening sweeper.
Edit, actually, come to think of it, they can easilly be compared. They have lacking defense stats, require good team support, and are weak to stealth rock.
Note that, yes they are comparable, but Weavile isn't very good, I don't know why you want the comparison. Also, Staraptor doesn't have enough power to support an agility set only. His defenses are so lackluster that all you need is prio to wreck him, and you don't have sub to protect him from it on this set. I don't care how fast you are, Prio will get you. Just like it doesn't matter how strong SD makes weavile. Both of them are made of Paper Mache, and crumble easily. The only way it will easily clean up late game is against highly offensive teams, who don't have defensive pokemon to fall back on.
SD Scizor +0 Bullet Punch: 60.1% - 71.4%
NP Infernape +0 Vacuum Wave: 40.8% - 48.2%
SD Lucario +0 Extreme Speed: 47.3% - 55.9%
Just to name a few. Now, Wall wise:
Bronzong Standard Tank Gyro Ball: 58.2% - 68.8%
Physically Defensive Skarmory Brave Bird: 46.3% - 54.3%
Physically Defensive Forretress Gyro Ball: 51.4% - 61.1%
Physically Defense Hippowdon Stone Edge: 68.2% - 80.4%
Support Cresselia Ice Beam: 60.5% - 71.4%
Staraptor against them. All calcs are for the same movesets as the above pokemon.:
Staraptor LO Close Combat vs. Bronzong: 39.1% - 46.2%
Staraptor LO Close Combat vs. Skarmory: 27.5% - 32.6%
Staraptor LO Brave Bird vs. Forretress: 41.8% - 49.4%
Staraptor LO Brave Bird vs. Hippowdon: 40% - 47.1%
Staraptor LO Brave Bird vs. Cresselia: 34.5% - 40.5%
How are you going to clean up against any of the common physical walls out there? They would have to be nearly destroyed completely in order to fall to this set. They'll kill you before you kill them, even if they are wounded. Not to mention Skarmory and Hippowdon can easily heal off your hits. I see no merit in putting in all the team support this set requires, when every single one of these pokemon stops it dead, just to name a few. Also, You can't say "well I'm gonna hit them on the switch in" because Staraptor has to use Agility on the switch, otherwise it will never get the chance. I'm gonna repeat my statement than Agility should be mentioned, but doesn't deserve a dedicated set.
EDIT: Added more Calcs