As to why they're pressed to one side, I've always heard it told that it made it stronger once you've quilted it as if pressed open the seams could come apart from stress. Also stitching in the ditch with opened seams I would think wouldn't make the pieces any strong and you may cut the threads as you're stitching so making it weak. I've never had issues quilting with 1 layer more going across the seams either but that's just my input.
There are some times when you find the instructions state to press open and I'm sure there is a perfectly good reason. When you have more than 2 seams, you might want to fan or twirl the seams to spread the seams out which I find so much better than a lump in the middle where they all join together. I salute the person that came up with that idea.