What if it is still 1-on-1 communication; so, imagine this: I use an app to write the email, select a group or set of contacts, and the system sends out a 1-on-1 style email to each one of those contacts?
It also will shift the burden solely to you to answer any questions or provide clarification. If recipients were able to see other recipient responses, they could do some of the work for you.
But, for all the use cases I can think of with regards to sending messages to multiple recipients, I cannot really think of a good reason to prevent recipients from seeing other recipient responses barring some communication of a personal nature.