Burn Marks and Rust are Not The Hallmarks of Great Grill Grates
Cast iron fry pans and griddles can become non-stick with use, so many people have the misimpression that cast iron grill grates are also non-stick. They are not. Pans and griddles become non-stick because, believe it or not, the metal is slightly porous and the surface is textured, so oil nestles in there and forms a slippery coating that can last for years if it is not overheated or scrubbed with abrasives.
The problem with grill grates is that (1) they get superhot and (2) they get scrubbed with abrasives all the time. They get a lot hotter than most fry pans and that burns off the oil, turning it to carbon. No more non-stick surface. Then, when you are done cooking on a grill, you scrub and scrape off the grease and food residue along with any non-stick properties.
It's true that cast iron grates are heavy and have a high thermal capacity, making them very efficient at and holding and transmitting heat to meat. They make definitive grill marks. But deep grill marks don't translate to maximum flavor because that leaves much of the meat surface (and potential flavor) undeveloped. Click here to read more about the myth of grill marks.
Worse, cast iron grates need babying. You have to scrape them immediately after cooking, then oil them while they are still warm. The oil will fill the pores that have opened during cooking and prevent rust. To do it right, you need to run the food into the house, leave the fire on in order to burn off residue, and while everyone is waiting for you, run back out, scrape them and paint them with oil. If you forget, within days they can start rusting and that stuff is a pain to remove. If you don't use cast iron grates for months, they will rust even if you oiled them after the last use. If there are long gaps between uses, you really should clean them and bring them into the house.
Then, when you warm up the grill for your next cook, that oil becomes stinky smoke. And whatever you do, don't drop one a heavy cast-iron grill grate on your toes.
Frankly, I think they are more bother than they are worth.
Some manufacturers sell cast iron with a porcelain coat. These are a lot easier to care for but they suffer from the same grill marking problem as cast-iron grates that are not enameled with porcelain.
Here is what my friend, Clint Cantwell of Grillocracy.com, does with his cast iron grates.