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?
Your original premise, as I understood it, was to send a message to a group of people and possibly have people comment on it. With your new proposal, the recipients would not be able to see each other's responses.
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.
I think it'd be interesting to have an option for either type. I find a lot of commentary is suppressed because people don't want other people to see their thoughts, whereas with email, they can just reply directly with thoughts. However, I think you're right, that sometimes it's useful to have a group discussion. I'm thinking that would be a link at the bottom of the email that goes to a webapp that allows a more FB-like experience that people can opt-into.
Email really should just be used for 1-on-1 communication that's not meant to be read by anyone else other than the sender and recipient.