I think that sand, lava etc should be allowed with world edit, as it is incredibly helpful with builds. I know its banned because of lag, but perhaps there should be a limit for sand/lava that is separate to other blocks? I don't know, its just a thought.
The amount of lag is already high enough, and if those blocks will be added some players who want to do bad things to the server or lag it will profit. And with a limit or not, the lava will still cause lag.
There is a command (//ifixlava I believe) that will make a pool of lava from the block you are standing in. As for sand, you can stack it, which I know isn't too helpful but still.
