I tried to fmq with my Janome 6600 without the bobbin case and got really bad results. Popped in the new case and got really nice stitching.
A friend of mine purchased her 6600 at the same time I did. She doesn't need the fmq bobbin case. So I think it just depends on the machine. But I do love the case, but not too excited about the "hopping foot" they sell. Mine doesn't hop.