I thought the whole idea of the multipool would be to pay out solely in slg

Well the effect is still the same as long as the miners who are part of the Sterlingcoin community mine at Cystum, then the Sterlingcoin price will be supported*. However the work done on the multipool and the way it functions would be wasted if it was simply used to mine coins and payout in Sterlingcoin. By supporting multiple coins for payout we will attract more miners, interest and in turn we hope the awareness of Sterlingcoin is raised and more people wish to be paid in SLG and to be a part of our community. Cystum also provides a fair and safe place for people to mine with the assurance of having public developers with trusted previous work. Cystum could be the answer altcoins are looking for to go against pools such as CleverMining and other such multipools that mine profitable altcoins and dump out for BTC.
*Dependent on hash and other varying factors