The custom is when someone is being Yotzei in a Mitzva from someone else (such as hearing the Shofar or reading of the Megilla), the person listening does not answer 'Boruch Hu Uvoruch Shemo' to the one making the Brocha. I assume the reason is that this would constitute a Hefsek - interruption between the Brocha & the Mitzva the Brocha is going on. However, I have noticed people who refrain from saying 'Boruch Hu Uvoruch Shemo' during the Chazzan's repetition of Shemona Esrei. What would be the reason for this? The Congregation already Davened their own Shemona Esrei so they aren't trying to be Yotzei with the Shliach Tzibur?