You're assuming special messages are extremely rare, but a group could make use of them more frequently. Say, a group that produces a weekly newsletter. Some people just want to get the newsletter each week, and some want to chat about it.
- Users who want everything in mail, the newsletters and the commentary, just subscribe to all mail.
- Users who just want the newsletter in mail could subscribe to special notices, with the rule that a moderator will always send out the newsletter as a special notice. Only the newsletters (and really important stuff) get sent as special notices, so those users get what they want.
- Users who don't want a weekly newsletter in their mail but want to read it online set their accounts to No Mail.
So that would be an example where No Mail and Special Notices would both be realistic settings for different users.