Detonations are bad in of themselves, they cause various items to become contaminated, causing issues for imaging and science applications. Being able to ignite

Detonations are bad in of themselves, they cause various items to become contaminated, causing issues for imaging and science applications. Being able to ignite fusion without using a fission bomb to do it lets you prevent the contamination without sacrificing your negotiating position. Remember, the best lie is the truth, your enemy can't figure out your nuclear weapons don't work if they actually work.