I have a stash of inherited family quilts, some over 100 years old. I've noticed that the heavier quality muslin used in many, both within the top pattern and for the backing, holds up better through time than the printed fabrics.
I like the muslin for block backgrounds, etc. It gives the rustic, older look I like in my quilts.
However, I've also discovered all muslin is not alike--texture, density, and even color, whether the bleached or unbleached. So--if you are using it, be sure you buy enough at the time for your project or you may not be able to find the exact match later. Luckily, when I decided to increase the size of a quilt I was making using muslin on hand and couldn't find a match, I found some at an estate sale that was an exact match. Was I happy when I got home with it and compared it to what I was using that it was identical!