Wrong again, Kuja. He can have the Soul Reaver active for as long as he wants, it doesn't just start draining his life for no reason. It only starts feeding off of him when he's killed too many enemies with it and it goes energy-crazy.

And he constantly regains energy by simply being in the Spectral Realm. Just by standing in the Spectral Realm, his life slowly replensihes. He can absorb souls while there also, because the Reaver functions differently in the Spectral Realm. The Reaver is less powerful in Spectral, and he can't use any of the elemental Reavers, but the it doesn't steal souls from him while he's there either.