Is Galera Arbitrator really required?
If you have minimum of three active nodes in your cluster, you don't need to add garbd.

You should use a garbd with the following conditions:


When you have an even number of nodes and you can't add new nodes (because of cost or ...) to avoid split-brain situations. 
when you want to backup data (SST method) to a server.


More information:
http://galeracluster.com/documentation-webpages/arbitrator.html