I prefer the crust you get over the entire surface you get when a using cast iron pan over grill marks. I do love using my CI grill grates mostly for the heat retention that mass of cast iron gives you. Me personally I have no use for the grill grates, not interested in those grill marks and I do mostly indirect cooking so I don't have flare up problems. Can't think of any other advantage grill grates give you.
Then they probably wouldn't give any advantage to you.
I, on the other hand, love 'em. Grill marks aside, I still do a lot of cooking over direct heat, so there's that. Plus, their design does amplify the heat of your grill so you can use less fuel.
When I'm at my parents', all they have is an old, small, cheap, Char-Broil gasser. The original grates were in pretty bad shape and a set of GGs fixed that problem along with the flare-up issue - got 'em on sale to boot.
Oh, and you just flip them over and they're great for cooking potatoes either wedges , rounds, or cubed up w/onions.
Finally, they've stopped the sacrifices to the asparagus gods.
Of course they're not the "be all-end all" by any stretch, and they're certainly not for everyone. And there are many other products, CI pans, CI grates for example, that are just as good, if not better.
But GrillGrates are a well thought out, well produced, product that is of benefit to many, many, many of our BBQ brothers and sisters.
BD