I've tried saving various items.
I wouldn't use interfacing. I think a sewn in interfacing wouldn't give the strength you need if the quilt has serious wear-through in a lot of different places. And an iron-on interfacing a) usually comes off after a number of washes and b) makes the fabric very stiff. Featherweight iron-on interfacing wouldn't be so stiff, but the featherweight interfacing is very flimsy and would probably just pull apart (it's the non-woven variety of interfacing I'm thinking of).
I once repaired a family favourite toddler dress that had threadbare elbows. I used regular broadcloth inside. I used my machine's darning function to adhere it to the original fabric in the worst places (weft threads completely worn away, only the warp threads remaining). Then I used a free-motion stitch to bond it to the surrounding weak area that wasn't quite as badly worn. (Too do the machine darning off the whole thing would have made it very, very stiff).
I've always thought it might be good to use that stuff that you see ironed on to knit clothes over the embroidery. I don't know what it's called. It is a white fabric that's iron-on but soft and flexible. It's a knit, so would be a lot stronger than non-woven interfacing. You see it ironed-on the inside of kids clothing especially AFTER the cute embroidery has been done. They use it so that the rough threads of the embroidery aren't irritating your child's skin. I wonder if it might be cost-prohibitive, though, to use it for something as large as a quilt.
