FWIW, I think some sort of minimal missile defense system to take down one or a few missiles launched by mistake or a rouge commander* might be worthwhile. Otherwise not so much. To easy to deliver nukes otherwise from what I understand.
*Supposedly this has come close to happening: a drunk Russian general supposedly ordered a launch but fortunately, his subordinates decided to call Moscow first........... Thank G_d they were Russians and not WW2 Germans
