I have a bernina 004D and love it... you are right you don't use them very often but... I have made a king log cabin with it..don't ever run out of bobbin thread!! and if you make a lot of bags.... they are great for finishing the seams on bags.
They are wonderful for the Kay Wood 9 hr quilt for babies all done on a serger and I've made several of those and some I have used wooley thread on the edge and no binding...
Judy