I play terran a lot and I use nukes all the time.  Especially in 2v2.  I'm only silver league in 1v1, but I've never had someone use a nuke on me once, except for friends.  In a few hundred games.  It's alot of fun, and its definitely pretty effective once you learn how to micro the ghosts.  I like to get 2 ghosts, 2 nukes, and used it to obliterate a big gateway or barracks wall, then rush into the base with the rest of my force.  Sometimes does blow up in my face though heh.

It's not cost efficient, easily scoutable, hard to land, and punishable.
 
When you're relying on nukes you're relying on your opponent being bad, not having enough detection and not having scouted.

From what I've seen in the GSL, most players use a nuke to destroy armies and use ghosts to EMP targets with energy.

TLO uses them a lot.

Some people do use them, it's a bit difficult though, since they are costly and difficult to land. Personally I see nukes as a form of harassment, not a way to win the game. This means that you have to be able to control an army and land nukes at the same time, since if you just focus on the nukes, it will be extremely difficult to make it pay off. Most people in lower leagues can't do that, so most of the time only better players who like to be creative try to incorporate them into their builds. 
 
Pro players use them only occasionally, since they are difficult to land against a competent player. I think they'd be super useful in 3v3 or 4v4 though.

I find it hard to get there if I'm in the middle of the battle. I do see them though, specially in long 4v4 games.

I don't usually play with sound, so they kick my ass when they are used.  Thankfully that is very rare. 

I wouldn't want to rely on nukes, but I think they can be used more than they have been especially in TvT. How about instead of getting into a tank war, go ghost marine marauder nuke? Put a ghost just out of range of the tanks, aim a nuke as close as you can, and they either have to move or rush marines to kill the ghost. If they move, you can cancel the nuke, run up and kill some tanks. If they send their marines up to kill the ghost, you can just mass snipe with your other ghosts and get a bunch of kills. Also, nuking a Terran wall just sounds like profit. Or, after a drop, aim a nuke at their ramp from the inside to buy a few more seconds, sort of like a forcefield.

Basically, I think nukes could be useful if you have a bunch of ghosts. So think of ways to use a bunch of ghosts.

Edit: seems I can't remember names

I blame Modern Warfare 2, gave nukes a bad name.

It's easy to prevent unless the player using them is leagues better than you, and if it doesn't do enough damage you're screwed. So they're rarely used because of that. I see them more in 4v4 where people can get away with more odd strategies like that.

Nukes are oddly enough hard to land. Most of the time players normally need to find a good location to nuke and many resort to nuking the mineral lines, which is the first place most competent players will check in the first place. As such, its hard to land a nuke since it needs to do enough damage for its worth. Of course nothings stopping you from nuking constantly in late game if its a split map situation (see MVP vs Creator game 3). I think they would be superb in TvT when you land a nuke near their tank lines and force them to unsiege or when you make a doom drop in the main and nuke the entrance to prevent their army from coming back.

I use nukes to force siege tanks to un-siege. If he doesn't see the dot, all the better.

Back in Bronze league I used them constantly to take out workers. Someone would lose their whole economy without even noticing.

Can't say it works much in higher leagues, but if I have ghosts with cloak I might as well get a nuke. You never know when it'll come in handy.

I have played 250 matches in ladder during these two seasons.
Only two of the matches had nukes in them.

It depends on what kind of game it is you're playing; but late-game, I find nukes are almost essential for good Terran play.

It depends on what kind of force the enemy is using, ofcourse; should you be facing off against a siege-tank/thor spam, then a nuke or two will win a game, same goes for any larger ground-zerg user. Bugger using high amount of air-units, though, it gets tricky, and nukes aren't neccesarily favourable. Most of the case any player who pays attention will be able to avoid the nuke in-call: but here's the trick to it.

Late-game there tends to be quite a lot of units on screen, and when the larger armies face off, it's just all one can do to hold the line and micro properly. Call in a nuke at that point, the enemy won't notice it 70 % of the time, and have his entire army wiped out; or he'll notice it and begin to pull back, which if you placed your nuke right, let's you chase after him, and while he's busy retreating, you can kill off enough of his units to decide the conflict in your favour. Late-game, I tend to enjoy battle-cruisers, enemy focused on that, most of the time he just doesn't see the nukes (for an almost guaranteed hit, use a second nuke in his retreat path that you launch just a moment after the first.)

Or, you can just use 'em to destroy all the workers in an expansion.

Nukes are used all the time.

They are incredibly good at stopping an enemy from retreating also. When the enemy goes to poke the front of your base with his huge army, you can throw a nuke down behind him and if he tries to retreat boom. Pros do it all the time.

Also people nuke my expansions a lot of the time... its rather annoying. If you can pull off your workers in time it isn't very effective though.

I wouldn't want to rely on nukes, but I think they can be used more than they have been especially in TvT. How about instead of getting into a tank war, go ghost marine marauder nuke? Put a ghost just out of range of the tanks, aim a nuke as close as you can, and they either have to move or rush marines to kill the ghost. If they move, you can cancel the nuke, run up and kill some tanks. If they send their marines up to kill the ghost, you can just mass snipe with your other ghosts and get a bunch of kills. Also, nuking a Terran wall just sounds like profit. Or, after a drop, aim a nuke at their ramp from the inside to buy a few more seconds, sort of like a forcefield.

Basically, I think nukes could be useful if you have a bunch of ghosts. So think of ways to use a bunch of ghosts.

Edit: seems I can't remember names