If you don't use frequently use them, they rust or grow mold if there's enough grease to prevent rust. (Maybe not; see below. But in GA it was a given. For me.)
Now let's say you want to press that extra grill or two into service. You need those grates because your daily cook grate will be used on your go-to cooker.
How much effort do you perform to scrape off the rust to an acceptable level, or burn off mold?
For example I picked up 2 machines last week. Both have rusty but usable cook grates. I want to throw hot coals in, and scrape off some rust, and grease them, with or without cooking food on them.
But why bother, when the cycle will just repeat after I ignore them and cook on my regular grill grate?
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a partial clue:
A third grill I just got is an AD code 26. Owner said it had just been sitting in his garage, unused, for like 2 years. The grate looks "ideal," lightly covered in grease but zero mold, zero rust. How??
Even if this is an SS grate (I don't remember when 26ers began coming stock with those) why isn't there mold? TN gets its share of humidity!