We'd end up with dual SMTP/SMTP2 systems
Indeed. We went through something akin to that when ESMTP came along. As an example, trying to send emails from a Solaris server to a server using Exchange 5.5 would fail silently since the Exchange server would advertise as ESMTP-capable and then silently ignore any ESMTP commands and drop any emails sent using pipelining..
Took me a while to find out why emails were going astray. Telling sendmail to *never* use ESMTP to the specific E5.5 servers solved the issue.
Now, it could be argued that the problem was Exchange 5.5 having an incomplete ESMTP setup (which was true) and the default being ESMTP was enabled (which was true) but it neatly shows how having multiple SMTP standards or extensions can be a problem.