My thoughts are you can never have too much weaponry, was the Doubtless the lead ship regarding the testing\evaluation of Viper? Could it be that complacency dictates that the system is so good it does not need CIWS? Viper may well be the all singing, all dancing latest, state of the art system but Murphy's Law will always out trump your best hand and when you need a viper to send a message, Mister Murphy will intervene and that to me is when you NEED CIWS secreted up a sleeve.
The story of the Repulse and Prince of Wales in the fall of Singapore bear this in mind as the belief was they were unsinkable.............Japan figured otherwise.
Complacency always a bad place to start from.