I'm wondering about not being able to drop the dogs. On my 503 the needle plate lever by the pillar has a "darn" setting, which, although it doesn't drop the feed dogs, lifts the needle plate above the maximum height of travel. Would this raised up plate interfere with quilting?
I have used the feature once, when I made a heavy blanket for my daughter by sandwiching one of those polyester blankets from the 1970s between two patterned bed sheets. I used the smallest cam on a button holer to tack everything together. I will say the button holer did kind of wobble a bit because of the raised plate, but it worked.
I also know that if a person takes apart this lift lever and doesn't put it back together correctly, it won't function properly (ask me how I know this).