I personally think that having 6 working gears on a trials bike is pointless. These bikes are designed to get over obsticles. Everyone I know only uses one gear to ride with.
I think if comp organisers ditched the gears we would see companys coming up with much better chain tensioners that are more robust reliable and safer.
You could say mechs work well to tension chains so why change it? Well to that I say XC frames worked for riding trials so why change that? Its not exactly the same but things move on.