Yeah, I've posted on this one before, IMO very dangerous functionality, would love to be able to turn it off.
I've had the situation where my FD built a report showing all the annual totals for the P and L double what they actually were. He used the column header "2009" instead of "Yr 2009", instead of key erroring he got back the result times two since 2009 was a subset that contained all the periods and the annual total.
He was not very impressed and it took me an age to find what had happened. I can see in some circumstancies that it's useful behaviour but I don't think it should be the default as I think it makes it trivially easy for users to report nonsense. One of the main reasons products like TM1 exist is to keep control of the business logic, this drives a horse and cart through system integrity.
The only valid reference for a DBRW should be something that is explicitly defined in a dimension.
Cheers