Actually I am sure I saw Marks use his dado head to do a tenon the
other week...

Why is this? Is there an advantage to loose tenons that I am unaware
of?
Does he just like to show off his multi router? Are they just easier
to
make

It appears that they are easier to make if you have already dropped
$2700 (plus the price of the router) on the multirouter.

The other advantage is that you do not need a special bit to the
tennons on rail and stile doors. For the windows I am making at the
moment loose tenons would not provide enough strength, but for cabinet
doors where you don't need as much strength and for entry doors where
you have a lot of tenon surface area this is is not as big an issue.