Two further thoughts from sleeping on it. Nightmares perhaps. Folder management and the meaning of Delete.
The original Gmail view of the Inbox was an an ever-growing stash of messages w/o folders or delete. They believe(d) that search is a better way to find things than keeping a tidy inbox and folder tree. Eventually they relented to user expectations and label/folder duality ensued. So I'm fairly convinced that giving users a personal inbox will bring on the expectation of having folders to stash things in. I'm not sure if that expectation can or even should be resisted.
The other part of keeping a tidy Inbox is Delete (Gmail eventually relented on this too). This one I think is far more likely to cause trouble. Not that it is a lot of work to implement, but that it has a far different impact in one's Inbox than it does in a group's Messages archive. It was rare, but there were occasional instances (in Yahoo Groups) where a newly appointed moderator would "catch up" on the group's messages, deleting them as they read them. Oops. It was a conceptual matter of mistaking the group's Messages for a personal Inbox. Having personal Inboxes within the Groups.io interface breaks the very strong distinction between personal content and group content in a way that might lead to mistakes of that sort. Everywhere the help or user manual (or GMF) talks about deleting a message there would have to be clarification about where you are deleting it from. Whereas now there's an inherent clarity to the concept that everything on site is group content.
That last is enough for me to think it may be better to take the Easy Way out.