See lomapaseo's note above - it's not just 4-eng aircraft or wide bodies that risk ingestion from FOD; all aircraft are at a greater risk of FOD ingestion when on taxiways than on runways for the simple fact that they are not usually as well-maintained/inspected in many parts of the world. Engine intakes on a low-mounteed a/c are much closer to the grass/dirt on a taxiway than on a 45m-wide runway. I take your point that there's little point adding fwd thrust if still braking, but by the time you vacate the runway you should be going slow enough that adding a little fwd thrust is not a threat.