I have not seen a definative thing that says a BSB is a REQUIRED re-roll, so if it is not required, then it is indeed optional with all the benefits that this might provide.
Per page 83 of the rulebook, under the section
RE-ROLL BREAK TESTS, "Note that the controlling player may not choose whether or not a unit is going to use the battle standard re-roll, but must always use it if the first test is failed."
The problem is that some people read the rules for Daemonic BSBs and mentally insert the clause "choose to" after "can" into the following sentence, "Note that an Instability test can be re-rolled if the Battle Standard is within 12" and/or tested on unmodified Leadership if the unit is stubborn." (DoC pg 30)
How does this ignore stubborn?
If the daemon player can choose to use the BSB re-roll (or not), then he can also choose to use a stubborn unit's unmodified Leadership. However, "choose to" does not appear in the sentence on pg 30, nor does that sentence specifically state that the basic BSB rules (i.e. mandatory re-roll for failed tests) are ignored.