The maintenance does kill it, but I could see a few advantages in the right (rare) situation. If you could handle the unhappiness from the population, gold-rushing these in a food-heavy city could easily make a trade route income that would offset the maintenance costs. Of course, that would come with the usual over-expansion drawbacks, including a cultural kick-in-the-pants to your social policies. I'd do it with the right liberty-based civ, but I'd NEVER put them in, say, a well-developed capitol... Which is a little strange. You'd think a city of a few million souls would want a hospital!