Later in the game it's not so much a culture bomb as a really powerful cultural weapon... it's true that you're not going to take a solidly locked city that way, but it's also true that it can provide a crucial boost to an already powerful culture producing city.
I usually have at least one serious cultural city on my border (hello Hermitage!) if I'm not going all out war to grab some border territory and maybe a city. It might not be as effective as a full out production city, but then its not as though you could have thrown a second ironworks into the city, and cultural warfare via hermitage + great artists can provide some serious leverage (I once actually won a late military victory that way, seizing a few crucial resources and enough territory to provide a great launching pad for a military campaign).
