I checked my logs of the last 7 days. My mail servers have sent emails to 29.401 different domains worldwide, 29.015 via MX records and only 286 via A records, which is less than 1 %. For a statistik about how many emails are sent via MX records compared to A records the result would even be lower. The missing MX records for the 286 domains are the result of lazy or clueless mail administrators.
While I think that deprecating A fallback would be a good idea, I don't buy that admins currently using it are lazy or clueless. What they're doing is explicitly permitted by the current standards.
Keith
