I don't know what the deal is, but +1 on them being terrible. I think they use a pretty low quality cast iron and the grates turn to crap quickly. My FIL has used his new grill with those grates less than 20 times and the grates are deteriorating pretty rapidly already.
I prefer cast iron, but I'd take a heavy duty SS grate over a light weight cast grate any day. You need a lot of thermal mass to get a good sear and keep the cooking temps up.
I bought a set of 4 Mangrates for my weber and placed the over my stainless grates, just keep them seasoned or take them off when not in use. I also use them on my GMG pellet grill when doing steaks / burgers and such. I pretty much only use my GMG these days
Mangrates
https://www.mangrate.com/offers/offers.aspx?affiliatesou
GMG Pellet Gill
http://greenmountaingrills.com/