You need to double fork() in order to ensure that the daemon won't be the controlling owner of the tty once it calls setsid(); i.e. that it will be re-parented to init.
So personally as a sysadmin that occasionally runs daemons from the command line I much prefer a double fork(); I don't want the daemon to exit when I log out.
Both forking and double-forking are completely wrong behavior for supervised daemons run as part of any automated system. Forking makes sense for small systems which lack any automated supervision and where a human admin is responsible for all process starting and stopping, but it's a bug anywhere else. Modern Linux has various methods for working around this bug (like the method of assigning an alternate process for orphaned descendants to be reparented to, which systemd uses, AFAIK), but really any production-quality daemon should provide a non-forking way to start it.
You have to fork, it's the only way to make a new process. Now exactly which process does the fork()ing is, somewhat, academic. Some might argue about the band aid approach of the requirements of parenting and process groups; but that would require leaving POSIX (and hence source compatibility) behind. Allowing any process to set their own parent, process group, or controlling terminal without very strong bounds is dangerous; kind of like having Erlang with mutability, it's possible but it breaks the model (and all the cuspyness that comes with it) and leads to bad juju.
Requiring init to spawn the final daemon process (and hence getting the pid from fork()) is a red herring, any process can get a list of its children and query their proc structure (modulo permissions, but most sane people run init as root).
init should be as small as possible to do only what it is required to do; this is simple anti-fragility at work. The less code and state the better. Feel free to stick your process manager, dependency calculators, and helper daemons in other processes. init should be able to handle something as catastrophic as a SEGFAULT and just execve(/sbin/init) and just carry on (while logging locally and remotely of course).
Yeah, I think we all know why the double fork model exists. It turns out, if fork a daemon from a different process than your login shell, you don't have a problem and you can more reliably track the process...
So personally as a sysadmin that occasionally runs daemons from the command line I much prefer a double fork(); I don't want the daemon to exit when I log out.