I see body bags as a luxury and convenience, not a necessity. Nice to have. Can help lessen the spread of disease in mass casualty situations. But not really essential for individual/family prepping IMHO. Might be OK if you have extra money to spend in your prepping. Body bags don't get rid of bodies. That's the real problem. Not the temporary storage of them.
Getting rid of bodies could be problematic. I'm not capable of digging a grave by hand for even one person. My wife couldn't either. We'd have to rent a backhoe and pre-dig some graves in our backyard, ready to go for when needed. That's a bit morbid and not confidence inspiring. I could see if one was serious about body disposal - maybe buy one of those cattle/horse metal watering troughs and some gasoline and matches to have on hand. Incineration seems like the quickest, safest and easiest route to me. I can imagine some pre-death uses for a trough in post SHTF scenarios too. So it might be a multi-use item (well, up until the first time you have to use it to burn a body). After that, using it to store water for household use wouldn't be high on my "want to do this" list.