An overcast stitch would be slower to sew, would add additional thread to the seam, and would be harder to un-sew if a mistake is made. I don't always know that I will want to press a seam open, so pre-planning which piecing stitch to use would not always work for me.
The only advantage I can see to using an overcast stitch to piece is that it might reduce loose threads in the seams. A machine straight stitch is plenty strong enough for piecing -- no need to make the piecing stitch stronger than that.