One thing I don't like about Bleach is that this isn't explained very clearly. But I think that when Rukia was explaining to Ichigo about why Quincy's were bad she said that when a Quincy destroys a soul completely he disrupts the balance of souls traveling to and from soul society. I took that to mean that when someone "dies" in soul society their soul returns to earth to be reincarnated. That would just make soul society a stopping place for souls waiting to be reborn.