It has been discussed before... Claiming in the nether means that its really easy to create wither killers. They are something that we have deemed to be endgame so it shouldn't be done in the first 5 minutes of the nether
Vanilla wither killers are easy anyway. Just go up to bedrock, place a piston, and push the wither into the bedrock as it spawns. It can't damage any blocks, and won't hurt you.
