I think if the bio parent is involved a step parent should not attend teacher conferences and such ever. IMO the main thing it shows is the step parent somehow doesn't trust thier spouse to take care of this type of thing appropriatly and wants to hover over them and "help" them make decisions on their child.
To me it is more a power thing for the step than what is best for the child situation. Especially in situations of a cheating adulterous spouse - who wants their Ex's cheatmate attending school conferences involving their kid ?