in all seriousness there are three good reasons to keep these around:

1, in the event of an actual nuclear war these are more likely to survive an EMP.

2, it costs less to warehouse these than to properly destroy them when you include all the paper work involved.

3, its always nice to have the old technology around in the event that the new stuff proves unreliable. for those keeping track, while both current warheads and missiles have been extensively tested individually, no one has actually tested the set together.