The bevel gears are Aluminium - so that means they won't last very long at all! Alu cogs mash up in no time on cassettes.
This was discussed and was said that becaue the meshing of gears is so precise, no large particles of grit can get in betweeen the gears. I am not so convinced. Surely a sealed unit or a rubber boot could have been designed to fit over the gears?
Another solution to a near non-existent problem.
Only time I cna see this would help is climbing up a loose steep hill where rear end traction is lost so the front wheel pulls the bike up (it will have the rider's weight over it).