I figure it's the best you can make of a mountain jungle. If you mine it, you lose its food. You can't lumber mill it. So build the trading post and if you're strong on Rationalism Policies, you'll get extra blue from it when you choose Free Thought.
When I played Civ 2 and 3, I was heavy into mining. I think even up to Civ 4, you could mine a grassland. But then I looked at some of the AI opponents when spying on them and realized that they all favor increasing population. It is overall the best strategy: more science, and with an Aqueduct you generate more citizens to work even more tiles.
So now my rule of thumb is: if it's next to a river or lake, it becomes a Farm. Only exception is if it's a resource tile. If it's a lowland jungle, I might farm it or I might do the trading post, depending on the city's situation.