What these two boil down it is a much simpler mail system architecture,
which in turn translates to a more secure mail system and an
easier-to-administer one.
Consider the control flow if you're trying to use port 25 for
everything:
Send a 220
If you see an EHLO, advertise that you support STARTTLS
If you receive a STARTTLS and another EHLO, advertise that
you support AUTH -- you don't want to do authentication
over insecure connections, especially if your goal is to
support roaming wireless users.
Accept inbound email. Check if the user was authenticated.
If so, permit relaying; also do rate checks. If not, don't
permit relaying, but do run anti-spam software.
Do virus checks. If authenticated, notify the sender that
either their machine is infested with *something* or their
credentials have been stolen. If unauthenticated, discard;
it's probably a joe job.
The point is that authenticated status has to be retained and checked
frequently.
If you're using 587, the subscriber flow is like this:
Send a 220
Don't accept anything until you see STARTTLS
Don't do anything until you see an AUTH
Accept inbound mail, do rate checks and virus checks, and
bounce accordingly
For port 25:
Send a 220
Optionally permit (but don't require) STARTTLS
Accept inbound mail. Do virus and spam checks, and drop
as needed. Don't permit relaying
Both are simpler; neither requires retained global state.