The numbers are correct, but your conclusion is not. 50% is the multiplicative inverse of 200%, so your attacks will in fact do twice as much damage against a target at -2. Any negative defense modifier will have the same effect as the equivalent positive attack modifier.Well, debuffs in general aren't too powerful to begin with.
First: -2 is not the negetive equivalant of +2. That's because buffs increase your stats proportional to your current value ...
150%, 200%, 250%, 300%, ..., [stage] * [value]/2
... while debuffs get weaker with every stage the target descend.
66.6%, 50%, 40%, 33.3%, ..., 1/([stage] * [value]/2)
For example: When using Swords Dance or Nasty Plot, you double your attack stats. When you now compare to Screech and Fake Tears, they only strip your target of 50% of their defence.
Really, the reason debuffs aren't used much is because they only affect one Pokémon, meaning you can't sweep with it, and it only lasts until the opponent switches, meaning it's easy to get rid of.