Who says not to use muslin? I especially love it for baby/kids quilts because muslin is very breathable & certain muslin fabrics are quite soft once laundered -- and get softer with every wash. I am working on a really lovely quilt (Calico Garden) with Aged Muslin in Beige by Marcus Fabrics for the background. Kona muslins are the same 140 thread count as their standard Kona cotton (and, if you love "painting", their PFD muslin takes ink really well!). I've also bought lovely dyed muslin from James Thompson & Co and Moda. If you look for the same qualities in muslin that you do in quilter's cotton, there's no reason why you can't use it exactly the same way.
It needles really well & gives a lovely traditional/vintage look to my quilts. I have never had any issues working with quality muslin. It comes in very few colors but the nice thing is that I can usually buy it at my LQS in 90 or 108"W. I've also read that it's best to steam muslin rather than press it because you can get permanent shiny spots on your muslin from pressing. I've not had that happen personally except when working with muslin that I've inked with fabric medium and tried pressing through a pressing cloth when my steamer broke (and that could've been due to the fabric medium -- idk). I've ironed/pressed all my other muslin fabric with no issue.
The two brands of "quality" muslin I avoid are Colonial & American Made. Colonial seems to fray more than the alternatives. The weave on American Made is a lot looser than Kona muslin making it more prone to bearding (I use W&P, which doesn't have a scrim -- so if I'm not careful, it beards quite easily). If you're going to use American Made muslin, I'd recommend quilting with either a 40wt or a 28wt thread and either an 80/12 or 90/14 Quilting Needle so that the spaces between the weave are filled in enough to prevent bearding -- & strongly consider either a bonded batting or one with a scrim.
In addition, there are 2 types of muslin that I would caution you against using. The first is actual
vintage muslin (like what you find on eBay or at an estate sale). Vintage muslin typically was a much looser weave. The fibers used in the weaving were also much coarser than those in vintage quilter's cotton (or in very rare cases, high quality muslin was much, much finer & more fragile than quilter's cotton). It produces a quilt with a rougher feel & that is more prone to fraying and caught threads during the quilting and laundering processes. The second problem muslin is
dressmakers muslin that you find at sewing stores like Vogue Fabrics, Mood Fabrics or big box stores like JAF, HL or some of the muslin at Hancock's. They are great for making dress forms & I've even used the thinner ones for backing damaged vintage fabrics (fused with Pellon 805) before I could use them in a quilt, back when I took commissioned work. But they really don't make for very soft or sturdy fabric either on the quilt top or the backing of a bed quilt. The cheapy muslins at JAF are still great for backing a wall quilt where roughness doesn't matter & they aren't laundered nearly as often.
But with a little bit of care & common sense, I think you will find that muslin fabrics may soon become you absolute favorite fabrics to work with.
