It’s halts progress and if you think your enemies are too slow, then make them faster, not make us not able to move. A stupid idea from beta that stayed a long, with no purpose now.
You can keep the walls but only if you change the respawn system so that your team always respawns in a reachable area. Too many times you get people wiping then respawning on the other side of the wall. This leads to a situation where it can take 20+ minutes to bring the troll down without a team causing the team simply quit from boredom and frustration. Fixing the respawns will also help with the problem of team mates respawning past the point of no return and being out for the entire game.
If they wanted to remove walls entirely though, they could add some kind of enragement mechanic for the bile troll and stormfiend. Give them increased speed and aggression the longer they go without being properly engaged. If players try to outrun the boss it will catch up to them quickly instead of completely blocking the way and halting all progress.