why is there a cactus in the chests? In the original single player Skyblock, making cactus green dye was one of the challenges. http://www.minecraftforum.net/topic/600254-surv-skyblock/ (look under the spoiler "Challenges and Rules") thats an answer
Once I made a cactus wall around my island, I collected all the cactuses from other players that I'd killed and made a cactus dome. This was good because if people tried to get in they got hurt. I think you use cactuses if someone is chasing you, to delay them you can turn around and place it in front of them.
I hardly ever play skywars, but I think the cactus would be useful for a trap. Something like a narrow pit with cactus at bottom that people fall into and cant get out of?
I love it when I'm right behind someone and they freak out and set me on fire by placing lava on my feet, and I just had a super golden apple!
I'm fighting with @Thrawt to get to 10th place but SHHH he doesnt know i'm competing with him. I guess if i tagged him he will know