Single pivot has less bearings to wear, less pivots to flex, and usually (and in this case) big beefy swing arms to resist torsional flex. FSR and other multi link designs are designed to provide a more tuneable suspension set up, however with the advent of the fifth element shock, you'll notice that a lot of bikes that were previously linkage based (not necessarily FSR) are going back to single pivot designs. On the other hand the FSR linkage is being used by every man and his dog, and works well for specialised and intense among others. But, more bearings and pivots, more potential for flex, more maintainence, possibly more weight.