The money for these sam installations could no doubt be used to provide more tangible benefits elsewhere, security or otherwise, rather than catering to hypothetical scenarios.
Costs will be tiny, the rapiers were purchased long ago and the soldiers manning them would be paid either way if they were there or not, every scenario is hypothetical until it happens the only real costs would be the fuel of transporting them.
Also as a side note there is no defence against a nuclear weapon a single ICBM can split into a dozen warheads far out of any interception range, MAD is the only deterrent.