Yeah it actually literally reduces the total payout value of the post. Adding up the USD value of the beneficiary reward and the USD value the author reward, the total payout value is less than if the author took all reward. It basically increases the % of the total payout that is paid at US dollar value of Steem, and decreases the portion which is paid out in a number of SBDs equal to that part of the payout value (so as long as SBD is > $1, beneficiary rewards reduce the total payout value of the post). So if an author is willing to send SBD, it makes more sense to do it that way.