Precuts come from many fabrics, if you bought all the fabrics for a layer cake(you would have to have at least a fat quarter or 1/3 yd of each) it would cost you more that the layer cake. If you are building a stash, precuts are not the way to go. If it is for a pattern and you do not want to buy a lot of fabric, precuts can be economical.
If I have a project in mind, I will buy a precut if it will work. At this point, I do not need more scraps.