We experimented with different casualty carrying methods. Drag handles, as others have said, are useless for actually dragging people. First of all, a fully loaded grunt is too heavy to drag in most cases. Maybe on a perfectly flat mowed lawn could you drag 250-300 pounds of grunt. On any other surface, no. Then there's the choking issue depending on the vest.
We tried using a length of webbing to sort of loop under a guy's arms to allow two guys to drag him without choking him, but without a conscious casualty aiding you in keeping the webbing in place, it just slides off after a while. And again, rough ground of any sort makes this really difficult.
And depending on how the guy is injured, dragging of any sort is really just going to make his injuries much worse.
I think the ideal, assuming you can't just grab a stretcher off a vehicle, is one of those crazy carpet dealies - don't know the correct word for them. Basically a heavy nylon sheet, ~72" by ~30", with three or four handles down each long side. Light, easily rolled and unrolled, and allows multiple men to assist in safely carrying a casualty.