Getting a heavy load of conventional precise explosives on a target anywere in the world within 30 mins or so sounds great but what happens if one of the nukular armed nations starts to track several icbm's comming in their general direction? They need to get warned in advance and trust the warning they get that its only convenional, and that it wont hit them.
A few years back we came close to a nukular war just because some russian agency had not forwarded the message that we launched a test missile from Andoya. Small mistake... could have been a disaster.
Arming some of the Ohio class with 154 cruise missiles was a great idea, but im not so sure about this.