Based on my experience with Oriental fragrances I prefer to wear them in cooler weather. I live in a very hot climate, and find that nearly all fragrances perform better in cooler weather.
As you stated, Orientals usually tend to use spices and other ingredients from the East, where it is often hot. But in the main, at least as far as I know, these ingredients aren't necessarily used in the East as perfumes, but more as incense.
Oriental perfumes as a category tend to be rather heavy and and dense and opaque in texture. They make frequent use of sweet substances like vanilla or tonka bean and rich spices. Some of them can be powdery. Many people find these characteristics not well suited for wear in hot, and especially humid, weather. They can become overbearing, even stifling, especially if overapplied. In general, they exude warmth and richness, and that's not what most people are looking for in a summer fragrance.
But there are several sub-categories of Orientals -- woody, spicy, fresh, floral, and so on. Some are lighter and less sweet and more comfortable for wear in warmer weather. I think the recent Eau de Orient variants of YSL's Opium were produced for this reason. Burberry's London is also very spicy but is leaner and more transparent than many Orientals.
No one says that you cannot wear Orientals in hot weather, it's just that most people don't enjoy them as much as they do in cooler weather. Try them, and if you are comfortable wearing them, then do so. There really aren't any rules. Wear what gives you pleasure.