The problem is that unread mail is not necessarily the same as "newly
arrived" mail. Since he keeps unread mail in each folder, the unread
count doesn't help him because it doesn't distinguish between unread
count and "new" count.
My guess is that Mozilla-Mail uses the "*" to show which folders got new
mail delivered into them since the last send&receive check.
Evolution doesn't currently have this functionality, but I suppose it
might be a nice feature to have. Feel free to add it to bugzilla as a
feature request.