yes and you are in right direction.
here is from my notes : thanks to @Giuseppe Larosa.
the meaning of the command is to prepare the router to be multiprotocol and not only to advertise ipv4 unicast address family that is the default.
So no bgp default ipv4-unicast
means = be multi address-families oriented and check the neighbor activate command in each AF context to see on what address families each neighbor has to be contacted/consulted.
During BGP session setup the capabilities exchange tells on what "channels/arguments" the local node would like to talk with the peer.
For this reason when giving the command all existing BGP sessions are reset to perform capabilities negotiation.
May be in newer IOS images you can reach the AF configuration without using the commanf but in the past it was mandatory to achieve multi-protocol BGP operation.
see
http://www.cisco.com/en/US/docs/ios/iproute/command/reference/irp_bgp1.html#wp1012800
