At level 90, the cannons in Isle of Conquest are completely useless. The most damage I've seen them do on a vehicle was 30,000. That's on demolishers that have 3 million health. It would take 100 direct hits from a cannon to take down a single demolisher. There are usually 4 demolishers. Taking them all down would take roughly 400 hits.
Meanwhile, the most I've seen a cannon hit a player for was 10 - 12k health. With that kind of damage, you can't use a cannon to kill a Warrior with Second Wind, even if he just stands there.
There currently appears to be no reason whatsoever to man the cannons in IoC, and no reason for the opponent to kill the cannons. If you're manning them, that's good for the opponent, because you're wasting your time, in the first place.
Without cannons, there is very little viable defense that can be mounted against Demolishers in IoC. This makes IoC even more of a mindless zerg fest than it would be with functional cannons.