If a sysadmin running Wheezy has configured spamd not to start, the
upgrade to Jessie (and systemd) will disregard this and enable the
service.  Please put a warning in the spamassassin NEWS.Debian.gz file
so that this is brought to the attention of admins by apt-listchanges.

Alternatively, the spamassassin installation scripts could read
/etc/default/spamassassin and disable the spamd service if required.
Even better would be if the unwanted daemon was in a separate package
that could simply be left uninstalled in the first place, but
presumably there's zero chance of anything like that getting in during
the freeze.