It wasn't- this was a "fix" made in i25 by Leandro.
The second stack removing the -def debuff was a bug that Paragon Studios never fixed.
Except now, with the -def not being able to be bypassed, the more global recharge you have, the more Rage is an active detriment to you ability to stay alive. Especially in high level content.
Doubly so with the new proliferated sets adding more defense-based sets to tankers and brutes, this really cripples builds.
I never understood this fear of Rage being overpowered, because at the point that you get to that level of power, you're in Incarnate/heavy IOs sets and everyone is a walking god at that point anyways. Also, SS being smashing damage, it needs all the damage boosts it can get.