CORBA24 — why was is_abstract added to structs in CORBA IR?

I was wondering if anyone can explain the rationale for adding the field
'is_abstract' to the CORBA::InterfaceDescription and
CORBA::InterfaceDef::FullInterfaceDescription struct types in the CORBA
2.3 Interface Repository specification. (I do know about abstract interfaces,
I am really looking for the rationale for breaking backwards compabitility).

Basically, a CORBA 2.3 client using stubs compiled from the latest IDL cannot
interoperate using IIOP with the Interface Repository of a pre-CORBA 2.3 ORB,
because virtually any client of the IR will want to obtain a
FullInterfaceDescription from an InterfaceDef, and a pre-CORBA 2.3 ORB will
not marshal the 'is_abstract' field for the description struct, thereby the
CORBA 2.3 client will fail to unmarshal the struct.

flagged as urgent....resolved