The incident with U.S. bombers flying across the country loaded with nuclear weapons by mistake, which was reported in Military Times, is just an illustration of the dangers of designs that mix together nuclear and conventional weapons, like the Prompt Global Strike plan. The United States has in place a number of elaborate systems (described by Hans Kristensen in his blog) that are supposed to prevent things like this from happening. Still, it happened. As Hans described it, "the Minot incident is the apparent break-down of nuclear command and control for the custody of the nuclear weapons". You don't want a break-down like this to happen in an operational environment, with ballistic missiles carrying nuclear warheads instead of conventional. I know that the probability of a mix-up is extremely small, but I don't think it should be tolerated unless it is exactly zero.