gpg-connect-agent connects to (launching if necessary) gpg-agent by default.
if you supply it with --dirmngr, it connects to (launching if necessary) dirmngr.
It should be able to do the same thing with scdaemon, presumably using an
--scdaemon argument (which would be mutually incompatible with --dirmngr)
Do you really want to use it? The problem is that you won't have a Pinentry and
some other minor goodies. The better way is to let gpg-agent route command to
scdaemon. For example
gpg-connect-agent "scd help" /bye
shows scdaemons help.
hm, if there's a guarantee that scdaemon will only ever be launched as a
subprocess from gpg-agent, then maybe we don't need it.
If there's ever any expectation that some other program will launch scdaemon,
then it would be nice to use the unified launch mechanism provided by gpgconf.
The only reason I can see that scdaemon is not started by gpg-agent are
processes which run before a login, for example from PAM. But then the
autostart feature probably not needed.