Nukes are not useless, nothing in this game is ever useless. There are just a lot of better ways of doing what the nuke would've done for you anyways.
For starters, they are ungodly expensive. After you've built your nuke, you realise that you could've built two modern armours (which aren't one shot wonders) for the same price. As well, you realise that one nuke is pretty pointless against your rival nation, who has SDI already. You will need more than one nuke to hit. Probability wise, 1 in 4 SHOULD hit. So you would need 4 nukes to get one hit on an enemy city. You can make 8 modern armours for this, and drive it up to the city, kill the defenders and take it (or mix it up and have combined arms for cheaper). Now your new city will be productive once it comes out of revolt (with some exceptions).
When your nuke hits, if they've got a fallout shelter, it will barely scratch the enemy. Even without, I have never seen a nuke kill all the units in the city. Now you still need to drive units into it to take the city. To make matters worse, once you have it, you need to parade some workers up to clean up the guck to make it productive. It also starts global warming, which creates useless deserts, many in your own territory no doubt.
So there's really no sane reason to build and use nukes. They aren't even a good deterrent weapon, like planet busters were in SMAC. Anything you could do with a nuke you could do better with units, with the possible exception of destroying and enemy continent without transports, and even then, there is no material gain for you, and the AI still has the cities standing.
Only real use I can see is for some psychopath to build a ton and launch them at some backwards Civ with no SDI just for kicks. But then you might as well play Civ 3 for that, it was much more gratifying to see nothing but a pollution covered continent.