Hell in a Handbasket has a good post on kinetic energy weapons, which I think are a pretty good idea, that also mentions the idea of replacing the nuclear warhead on ICBMs with conventional bombs.
Although this sort of quick-strike weapon might be useful in certain circumstances, like when we cannot secure overflight rights or don’t have any planes or cruise missile-armed ships or subs within range of a major target, they would be prohibitively expensive if used as a delivery vehicle for a few dozen bombs.
Also, launching an ICBM is bound to make some other nations a bit nervous, even if we call them beforehand and inform them that we’re just launching conventional warheads.
After all, what would we do if we WERE launching a few nuclear warheads? We’d call everyone and tell them they were just conventional warheads! Not to worry! They might not fall for it a second time, of course, but with nuclear weapons second times won’t be required.
Even if we wouldn’t be so deceptive, other nations wouldn’t be able to count on our honesty. Not when their survival depended upon it.