We had one for the Simpson crossing and I'd say it is handy for that type of terrain. What saved us though was chatting on the CB. On one occasion we were about to gun it up a big dune but a bloke on the other side heard us chatting, he was about the do the same from the opposite direction so lucky he heard us.
A flag would not of made much difference if we had met at the top as you really need momentum on the big dunes so it would have been too late if we had met at the top.