Conventional warfare.. you cannot send in nukes before your troops to soften up the enemy. Assuming you lose an invasion you cannot send them in after you pull your troops out.
You cannot use them for precision strikes.
You cannot use them over your own country should it get invaded.
Apparently they are a deterrent from getting nuked yourself. Are they though? Nukes blow up civilians. Civilians do not press the big red button. Crack pot lunatics do from some deep underground bunker. Would you really launch a counter-strike killing potentially millions of civilians. I wouldn't. You don't answer genocide with genocide.
The only use I can see is a small tactical nuke which could be used to blow up an enemy fleet at sea. Even then, launching the nuke in way that it wouldn't get intercepted I guess would be difficult. The implications for such a use would also be pretty profound.
In the UK the Government want to / will renew trident. The UK nuke system. I don't really understand why. The cost is in the billions. The weapon is unsafe, as well as expensive to protect and maintain.
I don't know what the current state of the UN is right now is but it seems to me a better solution would be to give the UN 50 nukes or whatever. The UN then becomes the deterrent. Pass a resolution. 'Those who use Nuclear weapons will be counter-nuked.' Other nations can then give them up.
The only danger with nukes is that only one country has them. They can then threaten others 'Do as I say or be obliterated.' Realistically though so many countries know how to make nukes I can't imagine it would take too long to rearm in this scenario.