I didn't read the above comments, but this is my opinion why they should be illegal. A Mob spawner is meant to spawn mobs, and the whole point of this is to kill the mobs, correct? Well if it's killing them in a non manual way, this is just unfair to users who kill it in a manual way. If Mob traps were to be illegal let me break it down what would happen. Economy > Garbage bin. Sure, maby mobs don't drop the best of items, but they do sure drop them, along with Exp, if someone were to just sit there for twenty-four hours straight, this would make the economy go down the drain, the main reason people would want these to be legal, is because they're lazy, and don't want to work for their money. It's the exact same as X-ray in a way, you're cheating the economy, in a fair/cheating way, maby it isn't as efficient as X-raying, but it's for sure the same. They need to be kept illegal, or again the Economy will slowly go down the drain. In the long run it's like this: People who want it legal - Lazy. People who want it illegal - Want to work for their money; and don't want cheaters in their way of doing so. Use advantaged that arn't technically illegal, like water that pushes the mobs down to you if you're so lazy. In easy terms, it's an economy, just like in real life, people work for their money, not get it handed to them for sitting in a chair all day.
This is my opinion about mob traps, I say that they should be legal, (unless the lag is too bad) becuase if someone has the creativity and right design to make an effective mob trap, they should be allowed to build it, as it is a design to make more money. You still have to work hard to figure out a design to really be effective to make money. Just my opinion, tho it probably has no relation to this topic whatsoever
since this thread seems to have alot of people saying what is and what not is a illegal mob spawner is would one that pushes them a few blocks over and down 1 block then go in to a small room about 7 blocks from the spawner were they can still see you and hit you but they cant roam around much rooms about 3x4
this wouldn't be such an issue if it wasn't so hard to find mobs that spawn naturally. if it was possible to wander the surface and kill several mobs every night then people would be able to get their drops and xp without needing to have a trap and they would have to kill them.