Hmm.  Perhaps the last sentence could be "... because changes to the local dict propagating to the local namespace cannot be relied upon to either happen or not happen".  That would make it less redundant, since it would essentially be referencing the previous statement in the specific case of the consequences of modification.

The original included the caution against modifying it, and I think it is valid because of the inconsistent behavior.  Perhaps it could be weakened to "it is not a good idea to modify"?