OK, we've a fair public discussion (across multiple subjects and lists), 
some more private discussion, and some actual testing.  Here's my 
checkpoint and summary of the discussion thus far:

(1) currently, out-of-the-box, post install, neither mailer runs.  This 
    is due to the NetBSD install requirement that no externally 
    accessible services run by default.

(2) there is *no* consensus on the sendmail configuration for merely
    sending daily/weekly root mail, without accepting incoming mail
    (the default necessary to install).  

(3) sendmail example in /etc/defaults/rc.conf does work with the 
    current sysinst, with some manual tweaking. 

(4) manual tweaking is undesirable.

(5) sendmail is better at managing large numbers of domains.

(6) the out-of-the-box install only needs single user mail forwarding, 
    handling only 2 to 4 messages per day. 

(7) sendmail has a bigger memory footprint than postfix.

(8) sendmail has a history of security problems.

(9) postfix does *not* have the localhost problem (PR install/21999), 
    because postfix always uses FQDNs internally.

(10) the change to use postfix without incoming mail is simple and 
    straightforward (1 line).

(11) those desiring to use sendmail for larger installations also have 
    the expertise to handle the mailer.conf change, and other changes.

(12) and most surpassing to me of all, most of the respondents, even 
    those suggesting the fixes for sendmail problems I described, 
    state they've changed to postfix.  The populi have spoken!

Therefore, I conclude that postfix should be the DEFAULT mailer, but 
not the only mailer.  That requires only 3 file changes:

 * comment (add # next to) smtpd in /etc/postfix/master.cf
 * swap the commented lines (sendmail to postfix) in /etc/mailer.conf
 * have sysinst add "postfix=YES" to /etc/rc.conf

This will not tread on anybody's toes, will not affect current 
operations, will not prevent folks from using other mailers, but will 
solve the current install bug (PR install/21998).

Someday, there may be system packages instead, but they aren't here 
yet, and nobody seems to be assigned to them.  Until then, this is 
the only viable solution.
-- 
William Allen Simpson
    Key fingerprint =  17 40 5E 67 15 6F 31 26  DD 0D B9 9B 6A 15 2C 32