Knocking someone prone (with shield slam for instance) can put them in a disadvantageous situation to give you fortune dice and him misfortune dice in any upcomming exchange.
However.
Getting up from prone is a maneuver, so it's not going to have much effect on the target. The round you attack with shield slam, he well be down and if he acts after you, then he will get up and gain no misfortune dice and you will get no fortune dice on the next attack. If he has already acted then any allied you have may benefit, but most likely he'll act before you the next round, getting up before you act. I know some tinkering with initiative could be used with benefit to act after the target one round and then before the target the next round.
It just seems being knocked prone isn't really something to be very worried about.