I think the plate can be used instead of dropping the feed dogs. Check and see if it fit OVER them when they are up. That way you don't have to mess with dropping them if you just have a bit of darning to do, and then back to regular sewing. I have not looked at the 306 - but is that adjustment to adjust the feed dogs (and you can lower them all the way) or like the 15 where when you tighten it back up, the feed dogs go back to where they were to start.
Sometimes the plate is also for straight stitch only (small hole) and when darning and embroidery, the smaller the needle hole is, the less lightly you'll end up getting the fabric pulled down into that hole.