I use mild steel in my grill grate builds for a couple reasons:
Price - Stainless is much more expensive. I just paid $510 for a sheet of 1/8" stainless and $128 for the same in mild steel.
Non-stick - Mild steel, when properly seasoned is a perfect non-stick surface. stainless is less effective at making nonstick without some sort of coating.
Cooking for the love of it.