Your approachs tries to reach a different goal. My goal was: I have an existing group of users and want to mail to them a simple e-mail. See it more as a replacement for bulk-mail than for something like forums.
Obviously your approach would work, if (as you duely noted):
a) You have an interface that lets you create a forum for a group
b) Have an incoming callback that allows you to mark an e-mail address to go to a forum, therefore changing the notification-xxxx email address, as this is not something a user will easily remember
But it forces you to deal only with users. But a group can consist of parties, which is a CRM use case I have been constantly banging my head against. So, to do this, I would have to either make all parties users (which is difficult to do if the e-mail address belongs to an organization...) or rewrite forums to allow notifications being send out to parties instead. Okay, probably just change notifications to support parties. But this will make your point 3.) invalid.
Simple use case: I have a couple of persons on my site which are in the group "moviegoers". I write them occasionally about new movies I want to go to. Previously I did this by manually selecting them in Outlook (Entourage) and send the E-mail out. Now I can save myself the trouble and just mail to moviegoers at my site.
But, if you could get a) and b) working and prevent "I'm on holiday" messages, I would be more than interested to follow your approach and implement the b) part, as it does have merits for a community toolkit. My main objection to Forum incoming e-mails has been so far the not easily remembered e-mail address and the fact that it did not deal correctly with vacation messages going into infinite loops.
