Bear in mind that in the case or out of the case - either way alone does little to protect a guitar from temperature and humidity. There is nothing magical about a case. After all, it isn't as if you can quickly shoo out all the unwanted moisture or heat before you close the lid.

Guitar humidifiers, such as mentioned above, or even a desicator (similar to those little packets found in some new shoes), depending upon your climate, can be of great help in extreme conditions.

If climate is not a major problem and the guitar is in no danger of being knocked over, then by all means leave it out. It is far more likely to be played. It is often difficult to hear the guitar calling you if it's voice is muffled by the case.