You don't "have" to cover or drop the feed dogs for fm work. I don't and have never had a problem. I just lessen the presser foot pressure and set the stitch length to zero. on one machine I have I don't even use a hopping foot I just use the regular ss foot. Originally those covers for the old machines were made for darning, which covers a much smaller area and you would generally move the piece much slower so the issue Cathy mentioned wouldn't really be that much of an issue.
I think if it works for you though, then it's a great idea.
Cari