As I added in the edit, it is lot less used then what a cursory look may lead you to believe. Mostly only SLBMs really use it.
Regardless, it does not change the number of active warheads a country has, so hitting one city multiple times means not hitting other cities.
Plus, it would take dozens to really cover a city like New York. You can play around with https://nuclearsecrecy.com/nukemap/ to get a feeling for it.
Edit: To be clear, many missiles are capable of carrying MIRVs. Minuteman III missiles can still carry up to 3 warheads. But due to the New START nuclear treaty limits, they are loaded with a single warhead.
As I added in the edit, it is lot less used then what a cursory look may lead you to believe. Mostly only SLBMs really use it.
Regardless, it does not change the number of active warheads a country has, so hitting one city multiple times means not hitting other cities.
Plus, it would take dozens to really cover a city like New York. You can play around with https://nuclearsecrecy.com/nukemap/ to get a feeling for it.
Edit: To be clear, many missiles are capable of carrying MIRVs. Minuteman III missiles can still carry up to 3 warheads. But due to the New START nuclear treaty limits, they are loaded with a single warhead.
Source: https://eu.greatfallstribune.com/story/news/local/2014/06/18/last-malmstrom-icbm-reconfigured-treaty/10773351/
https://www.af.mil/News/Article-Display/Article/485611/malmstrom-completes-final-minuteman-iii-configuration/