Stakes haven't been implemented because the only way to do it properly is extremely performance consuming and could lead to noticeable slowdowns, especially with many players. We don't want that, do we?
I'm assuming that this is because you want people to be able to move through the stakes, but not horses? Otherwise they are theoretically no different from siege shields as far as i can see.
If this is the reason for the performance issues, then would adding individual stakes to the game work instead?
So instead of 1 person placing a block of 10 stakes (that is awkward because people, but not horses, need to be able to pass through the block), 10 people would place 1 stake each. If they are close enough together nothing can pass through them but attackers only need to destroy 1 stake to make a gap. If the stakes touch then just like ladders they would destroy each other - not realistic but I'd assume this would help with performance.
This method also requires people to work together for sucessful stake deployement which is good imo.