Flew ARN-EWR the other day. Before landing you can see connection information. 12 flights were listed: 9 on CO, 1 on UA and 2 on US. There were probably more connections to be made than that but not listed for various reasons, but EWR is not and will not be a good airport for switiching to *A-partners unless you want to go to their hubs (DEN, ORD, IAD, PHL, CLT) as a final destination.