I'd second the front one for braking. I've got a totally independent rope goes from the bow eye on the rib back to the trailer chassis so it's already tight if the boat moves forward.
What the rear does is stops the back of the boat being impaled on the rollers by the weight of the engine etc should a pothole manage to separate them. Also should something really nasty happen and it rolls then there's only one flying lump instead of two.
As Daibheid says - the dynamic loading is huge - even a small rib will weigh around half a ton, and you've all seen the ads for wearing rear seat belts, and that's only at 30mph..... There was a pic in the Laser class association mag a while ago iof someone who managed to put their boat in the boot of their Corsa with an E- stop on the motorway- and those things only weigh 58Kg in full sailing mode and don't tend to sit on roller trailers!
On a more humourous note, do you think his truck has bearing savers?