I always thought the deterrent made sense but after the Falklands War doubts stated to creep in - if you don't use it , or threaten to use it, when someone wades into your country -or its possessions - and you just manage to retrieve the situation using conventional forces then you have to think maybe more the cash spent on the SSBN's would be better spent elsewhere on the military. Right now its very hard to see a scenario where it could ever be used.