Other points if you want more "ammunition" ;)
JWs believe that Jesus came to the earth to pay for the sin the first human (Adam) committed on earth. If Adam never existed, then why would Jesus need to come to the earth and pay for his sin?
It is also their strongest argument for a paradise earth. They cant really prove a paradise earth with the bible if you read it in context, so they like to use this argument:
God created the earth to be a paradise with the garden, Adam and Eve would take care of the garden and expand it until it surrounded the earth (paradise). But Adam and Eve sinned against God and stopped that plan. But nobody can stop God, he still has his plan for the earth and he will make it into a paradise like he originally intended.
The garde of eden needs to exists for this to make much sense...
I was actually told the Jesus argument by a JW when I asked why they could not believe the genesis account was a story and accept evolution.