Most defensive positions aren't tactically useless to my mind. Look at Totalize. Every defensive position (mostly flagzones) are very useful.
The trenches, As i've seen, are only used for sneaky people who go in them to hide to try and cap... not to defend.
That doesn't devalue his argument, as proper defenders will use the trench as well and flush out the enemy.
It's amazing how long you can defend a flag with a determined squad and the given defensive positions, you should try it.