The main purpose behind the MX records is the ability to specify machine(s) to handle a specific protocol for the entire domain, and also to specify backup mail servers (with different priorities.) That way, if one server fails you can still reach the next server in line to deliver email to that domain. Neither can be done with plain A records, which map directly a full name with a host.
It can now be done with SRV records (dated 8 years ago, not 25) as Frank points out. Back then there weren't many other standard protocols massively available.
