I'm not at all sure why it's popular. I am not all that keen on doing it, because it has the effect of not showing. Plus, if the seam allowances are both ironed in the same direction (i.e. towards the darker of the patches), then I find it quite hard to sew on the seam line anyway. If I do intend to SID, I often iron my seams open. Shock, horror. I reason that when quilters were advised to iron both to one side, it was probably in the times when patches were hand stitched, and at that point, were relatively weak. Now we mostly machine patches together, and in my case, I use quite a short stitch. Ironing them open is a little harder, but it is much easier to quilt over - you don't 'fall off' the higher side. If ironed open, it is also easier to hand quilt over the seams too. I don't seem to have any problem lining up patches either, though nestling in seams together when they are ironed both to one side is also good, except I often found that I had them ironed the wrong way to do that. Hey ho, life is ever thus.