It isn't just the maintenance. In a typical club, just about everything
that happens on the ramp will fall into the MO's lap. With a few co-owners
communication is easy. With 20 - 30 members there is a lot more
communication required. The PIC has a very high level of responsibility for
the condition of the aircraft when the wheels leave the ground. It isn't
practical to let 30 members go through the logbooks so keeping them informed
and up to date on AD's and other maintenance issues so they have a basis for
fulfilling their responsibility is a big part of the job.

Owner's are also far more responsible for the maintenance of the aircraft
than many (or most) realize. It isn't sufficient to just call the shop and
tell them to do whatever needs to be done and put the plane back on line.
The MO should be up to date on things like AD's, independently verifying
that the shop is doing what is required, checking logbook entries, etc.
This takes some time.

The owner / operator is responsible for the maintenance. The shop works for
the owner / operator. The owner / operator is required to supervise the
shop.
--
Roger Long