I don't want to unload my quilting machine from the frame if I want to sew something.
And different machines have different features - my big brother is a straight-stitch, super fast machine, so I piece with it because I only need straight seams and I want to go fast. My little brother is slower and has zigzag, buttonhole, and other decorative stitches, so I use it for those stitches. You really don't want to go too fast when you're stitching buttonhole stitches.
The only reason I generally have two machines set up, one for quilting and one for straight stitching is because I need a different foot and different tension for free motion than straight stitching. Sometimes I want to quickly go back and forth without going through all the mess of switching feet, thread, tension and all of that for just a few stitches.
Sometimes it is just a matter of what feels more comfortable. Over a yr ago I bought a Viking Ruby but mainly only did embroidery on it even though it's great to quilt on. I even took a class a few months before trading it in on FMQ and it was great. I used my Kenmore/Janome to piece on because I know exactly where the 1/4 in markings are and it just feels right. I have now traded my Ruby in for the Diamond Deluxe and my goal is to start using it for quilting/sewing as well. This weekend I used the new interchangeable walking foot to finish quiltint a runner and bind two quilts that I had in progress. The newest foot I have is the new SITD foot with a wide slot that allows you to do the fancy stitches like zigzag and others that needs the wide slot in the foot. It's great and gives a great touch to the binding. You can even use the twin needle with the new foot. I love the interchangeable walking foot that Viking has because it doesn't limit you like a normal foot does. The other thing that makes the Diamond better for quilting is that it has a larger harp and programs for a couple of different FMQ feet depending on what you like. I've seen on here though where alot of people like to use their old singers to quilt and feel good with that. It's more a matter of preference and convience I think to most, at least it is to me.
My Pfaff 4.0 for everything except PP, use my Brother, as I leave it set up for that.