Carriers are incredibly usefull! Especially in late warfare. For granted, they pale in comparison to real carriers, they server a great purpose.
Improvement destruction may be the biggest use for carriers. When I got into a war with China, they were a continent away, and they wouldnt sign peace, so I bombed the crap out of all of their "luxury" squares, thus sending their empire into a state of Anarchy. While I lost a few bombers in the process, I did have some to spare, but it hurt they alot.
A good strategy for carriers is to scatter them throughout the world, just in case. You dont have to put bombers on them, but they do serve as mobile air fields for the occasionally needed bombing.
ironfang