As Blaine said parallel are bulky. Open barrel are not but they are incredibly difficult to do in tight quarters compared to a butt connector.Another question for the group:
Why butt connectors instead of "parallel" connectors? If I understand correctly, a butt connector (whether split, brazed, or solid barrel) doesn't allow the two ends of the wire to come into contact with each other. Instead, the connector itself is the conduit between them. But a parallel connector, or an open-barrel connector, would allow the wires to be in direct contact with each other, even allowing for some solder if needed. Isn't a direct connection always better?