I like all but 3 should not be a premium/enterprise option. I would want it for my basic group which got transferred from Y! groups for 3 reasons:
1. There are over 700 members comprised of people mostly unaware of the mail server options they have and are used to things working with the Y! group construct.
2. While I can turn it on for my account so I can see my messages, this does fix the issue of other people's addresses failing SPF validation when they have not turned it on.
3. Many people create rules so if the message is from groups.io with the group keyword in the subject, the email is moved to a specific folder. Without that indicator, their could be private messages, for instance, being put into the wrong folder. Simply a convenience in my reason #3.
Also, #2, I think, should read "users from smaller sites -not- implementing DMARC" since, it is my understanding, groups.io automatically enables it when it sees DMARC configured for a domain.