Carriers were one of those things that I left on, but didn't expect more than one or two people to even consider. I actually don't use them that often, but in games where I need them, they are very important.
I think that if I ever played a game with Nukes, subs would be extremely useful. The thing is, subs come into play around the same time I can put all of those hammers into carriers and fighters rather than subs and missiles. Maybe I'm missing something, but it sure seems like I get better value with the carriers and fighters for my air support. Not only that, but the carrier/fighter combination gets better when I can upgrade to jet fighters.
If the damn subs had another use besides scouting and sneaking missiles up to the coast, I'd be all over them. It's why I make the suggestion that they should first have a useful life as anti-ship weapons before they settle into their missile carrying role.
I'm not sure if muskets are the least important unit, especially since I normally build them, but I could certainly get by without them. They are the first unit that ignore walls, but there's no way to make them city raiders. I like them as CG units in that period of time before I can field rifles but am possibly waging a war with cannons. They do a better job of holding the new cities than would longbows/pikes/macemen, especially if I'm an aggressive or protective leader.