The current acpid description:

Description: Advanced Configuration and Power Interface event daemon
 Modern computers support the Advanced Configuration and Power Interface (ACPI)
 to allow intelligent power management on your system and to query battery and
 configuration status.
 .
 ACPID is a completely flexible, totally extensible daemon for delivering
 ACPI events. It listens on netlink interface (or on the deprecated file
 /proc/acpi/event), and when an event occurs, executes programs to handle the
 event. The programs it executes are configured through a set of configuration
 files, which can be dropped into place by packages or by the admin.

i.e. no mention of systemd and logind while

https://www.debian.org/releases/jessie/amd64/release-notes/ch-information.en.html#idp943504

says:

  5.6.5. Interaction between logind and acpid

  ACPI events can be handled by logind or acpid. In case both services
  are configured to handle events in different ways, this can lead to
  undesired results.

  We recommend to migrate any non-default settings to logind and
  uninstall acpid. Alternatively it is also possible to configure
  logind to ignore ACPI events by adding:

  HandlePowerKey=ignore
  HandleSuspendKey=ignore
  HandleHibernateKey=ignore
  HandleLidSwitch=ignore

  to /etc/systemd/logind.conf. Note that this might change behaviour
  of desktop environments relying on logind.

If I understand correctly: If logind is used, then it is recommended
not to have acpid installed (assuming that logind replaces acpid
completely). Describing (shortly) the interaction between logind
and acpid would give useful information to the user when wondering
whether acpid is needed or not (in particular when comparing with
old machines).