feline fanatic, there actually is a ruler foot for the Innova, and a trapunto foot. Like you I use the open toe foot for everything.
I know that other brands, especially older ones, have had hopping feet that weren't tall enough to use rulers. I searched online and found this conversation about the APQS. I think it might answer your questions.
http://forum.apqs.com/index.php?/top...foot-confused/