A confusing experience

During my last migration to IRIS I met this Compatibility parameter

• NullSubscripts    true

0 - Throw error when referencing a null subscript.
1 - Null subscript references do not throw an error.

The consequences are shocking. This was allowed:

USER>zw ^rcc
^rcc=0
^rcc("")="***"
^rcc("",1)="*,1"
^rcc(1,"")="1,*"
^rcc(1,2)="1,2"
^rcc(1,2,"")="1,2,*"

But SMP had no idea what is going on. 

And after switching off that flag this was still partialy visible, but read-only

USER>zw ^rcc
^rcc=0
^rcc(1,"")="1,*"
^rcc(1,2)="1,2"
^rcc(1,2,"")="1,2,*"

I just don't understand.