No, I don't think that's a mistake at all. Even if a quilt was in absolute total tatters, I bet any of us would find something of it to save. A pillow, a little framed bit as a wall hanging, whatever. If you only have some diamonds to replace, YES! go ahead and pick a great fabric, prewash it and make a template. I'd use clear plastic and add that 1/4" seam for turn-under, then go ahead and blind-stitch those patches in with a matching cotton thread. If you find the job overwhelming or if it's just not working out, you can always have it done professionally. I love the idea that you'd like to give it a try yourself, Penelope.
One of our guild members (Sewsweet) has mentioned a helpful website and one of the topics on it is Quilt Repair and Care. You might get more ideas there! It's one of the topics in the Links and Resources thread, called "Lots of quilting information".