Many quilting books say to piece the backing horizontally to reduce strain on the seam. I clip the selvedge every couple inches on drapery panels or backing fabric. If you serged the edges together, the selvedge would be removed entirely. Piecing the backing has been done forever and I haven't seen a seam yet that has come apart, quilters know they have to make secure seams, no matter which way they go.
Carol J.