I've used 2X6 pieces several times to increase the footprint on a jack. I think it would depend a lot on the terrain you are in. Here where your biggest obstacle is a beach rut or a mud hole they could be useful. We've also used them to help fill the hole we were stuck in (jack the truck up and stick boards in the hole).
Several years ago (the first time I ever drove on the beach), I used a piece of drift wood on the beach to give my bottle jack a bigger footprint to help get unstuck. There are a lot of situations where they are better than nothing.
That being said, I broke two 4ft pieces(that had some age) this hunting season driving my golfcart up on a trailer so there is risk there as well.