I have Perfect Layers as well and I find it indispensable for what I do. I have a very nice set of rectangle dies from MFT, but they don't always give me exactly the size I want. Yes, you do start with a rectangle and then work outward, but you do not have to permanently glue your layers down. I often start with a rectangle, Dotto it onto cardstock and use my perfect layers to make another one a specific size bigger and then separate them. You can make any size you want.
And of course Spellbinders has plain rectangles in both the classic large and classic small, so if you get both sets you are getting rectangles in ever increasing 1/8 inch increments from 3/4 X 3/8 up to 3 and 3/4 X 2 and 7/8.