I've also clocked some serious off-road miles with Paul's fork mounted setup and I can say that unless you are racing on a single track, the weight is a non-issue. Look at many of the low budget dakar racers from the 80's and 90's and you'll see a ton of similar setups.
The frame mounted setups are better in a lot of cases, especially with full coverage plastic. I still like having the lights point where I point the bars. I still have nightmares from trying to ride my NX250 in the woods chasing some friends...it's lame light was frame mounted.