Kevin Tetz's Paintucation Body Panel Replacement DVD has a chapter on using adhesives for quarter panel repair. It was pretty interesting, he shows how to do it, and most importantly how to not have a seam show up later. The advantages are 1) no worries of sheet metal warpage, and 2) no worries about the metal around the weld rusting.
As mentioned above, the seam is visible on the inside of the car unless its covered with bondo or something else. But the same can also be said of any flanged patch panel unless the installer cuts the flange off and butt welds the patch.
That having been said, I prefer to butt weld patches, leaving no evidence a patch was made.