One correction, and i understand that there has been some confusion on this.

The steem that you get when you convert an SBD is not created when you convert it. Its created at the same time the SBD is created. That steem is added to the vesting fund (where SP balances are stored) . If it takes more steem to redeem your dollar (ie if the price of steem goes down) then the loss is socialized to all SP holders. The same with the gain if it takes less steem to redeem the SBD than was created when the SBD was.

SO if the price of steem went low enough, redeeming SBD could actually make your SP balances run backwards by depleting the vesting fund.