I'm sure at some level (setting aside the obvious desire to funnel public money into the MIC) there's planning for how future technology might make parts of the nuclear arsenal obsolete, and redundancy built in to compensate.

Missile interception is incredibly difficult now, but in 10-20 years? Submarines are undetectable now, but in 10-20 years (see the recent post about China developing new sub detection tech)? At least through the very limited lens of nuclear planning it makes some sense to give yourself different options to be flexible in the event one part of your strategy can be effectively countered at some point.