Floor pans question from a newbie

[grandpas66]

I have painted several cars and trucks but never repaired rotten panels. I am about to start repairing floor pans, rocker panels, one cab corner and one door pillar. My floor pans are rusted through along the edges, but the centers of the panels are good. I have seen several guides the suggest cutting as little as possible of the existing panel. can someone help me understand why this is preferred rather than to replace the entire floor pan? The entire floorpan is $10 more than the outer skirt (sold from Brothers) and it seems that the welds would result in more opportunity for weakness, misalignment and a beginning point for future rust. It seems that replacing the whole panel is a sound idea even though there is a lot of good metal there. If you have experience in this please point me in the right direction. Thank you.