Bodies are faster to throw or drag once zipped up in body bags, so I'm not sure why this is necessary.
Also, in terms of realism, do you know how difficult it is to drag 70-100kg of dead weight? I... don't, however I imagine it's not an easy task for the average person.
I suppose it would identify them as having less armour than patrol officers, which is something practical, however adding something because it looks cool and adds variety is a perfectly valid reason in of itself.
I think a weekly swap or maybe one week every four would make sense. If it was swapped for a few days every week, people who can only play a few days a week might miss it, or end up playing on it more than intended, etc.