I thought the group inbox was a filter on the user’s inbox which only showed you that group’s messages, and not the only way to access them.
It makes a lot of sense to show group messages in a user’s inbox.
When I’m looking at my inbox, I’m looking at a dashboard of things on which I have to decide: reply or archive. I don’t only want to see the messages for which I am the sole recipient. I want to see all the messages which are addressed to me – including group messages.
Think about it in terms of an email account. All new emails land in my inbox regardless of whether I am their sole recipient.
Some of our users belong to half-a-dozen groups and clicking through them to find messages can be tricky, especially as there’s overlap between some of the groups.