I'll quibble a bit with some of this, mostly the marks being the only advantage. A grill pan, properly hot, should work a lot like the Weber Flavorizer bars in their grills. Liquids and fats render on to the hot metal and are changed by the heat and rise as smoke and other flavors back onto the meat.
the difference is how much contact the meat makes with the pan.....and what you want the final outcome of the product to be.
a simple cast iron pan will produce a caramelized exterior like nothing else. It will also produce a fond which can be used to make a sauce.
a grill pan will produce the lovely marks. In my opinion that is the only advantage.