They REDUCE swarming and increase brood production some over a solid bottom board. I like them on my SBB because they break up the draft when the bottom is open, provide cluster space in the evening and still allow bottom ventilation.
I have them on most all of my hives. If I don't it's because I hived a swarm and didn't get around to putting the slatted rack on yet, or it's a long horizontal hive or a 8 frame hive.
Nothing is a sure thing to prevent a swarm, but they add to the overall effect of avoiding swarms.