Because it's a 'chance' so be no means guaranteed, to suppress a captain's abilities. It's pretty pointless (as are all the PoE weapon perks). All the PoE weapons are a big let down. Outclassed by the Trials weapons, which are much harder to get (the elemental primaries anyway). The fact most people are still using VOG/Crota weapons instead of any PoE ones says it all.