4 reasons
1. More Damage than a Controller.
2. Blasters can be Damage/More-Damage. I wanted to play something a little more interesting and maybe more pivotal on teams.
3. Regarding Defenders: When I play a Defender, I feel more... obligated... to take ally centered power picks. My defenders are a lot of support with a bit of damage. I like being a lot of damage with a bit of support. Plus I feel more free with my power picks. Heck, I may take one of those phase shift/intangible power picks if it fits my Corruptor. Which I would never do on my Defender.
4. Scourge is great! Oh, your cup is half empty? Scourge! There, now it's all the way empty.