I believe the reason Weber went with the porcelain plated steel in the firebox of the Summit was to make the grease flow more readily into the drip tray. The Genesis did this in part by having the large pull out drip tray made from porcelain plated steel. However, the cast aluminum firebox bottom on the Genesis can, in fact, be a grease catcher. It is a place where grease fires and bowed fireboxes get their start.
I would bet the Summit porcelain steel bottom works a lot better for as long as it lasts. I guess Weber figured that people who can buy Summits would be ready for a new grill by the time the porcelain steel started rusting through. I don’t agree with that line of reason, but it is my best explanation.